Light source in optical transmission system, waveform shaper, optical pulse train generator and optical reproduction system

ABSTRACT

The present invention provides a pulse train generator comprising: a dual-frequency signal light source for generating a dual-frequency signal; a soliton shaper for soliton-shaping output light from the dual-frequency signal light source; and an adiabatic soliton compressor for performing adiabatic soliton compression on output light from the soliton shaper, and also provides a waveform shaper used in this pulse train generator, including a plurality of highly nonlinear optical transmission lines and a plurality of low-nonlinearity optical transmission lines which has a nonlinearity coefficient lower than that of the plurality of highly nonlinear optical transmission lines and which has a second-order dispersion value of which an absolute value is different from that of the plurality of highly nonlinear optical transmission lines. Further, the present invention provides a light source comprising a plurality of continuous light sources of which at least one oscillates in a multimode; a multiplexer for multiplexing output light from the continuous light sources; and a nonlinear phenomenon producer for producing a nonlinear phenomenon on output light from the multiplexer so as to suppress SBS (Stimulated Brillouin Scattering).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit of U.S.application Ser. No. 11/134,275, filed May 23, 2005, the entire contentof this applications is incorporated herein by reference. U.S.application Serial No. is a Continuation Application of PCT/JP03/14925,filed Nov. 21, 2003, and further claims priority under 35 U.S.C. § 119from U.S. Application Nos. 60/428,001, filed on Nov. 21, 2002 and60/465,990, filed on Apr. 28, 2003.

TECHNICAL FIELD

The present invention relates to a continuous light source in opticaltransmission systems, a controlling method of the light source, anapparatus applied to the pulse train generating technique and an opticalregenerating system.

BACKGROUND ART

In the optical transmission system, as one of factors to limit the powerof light input to the optical fiber, Stimulated Brillouin Scattering(SBS) occurs, which then causes backscattering of a part of the inputpower and this may adversely affect the transmission system.Conventionally, the technique of suppressing the SBS include: (1)electrically controlling a light source; and (2) optically controlling alight source. The technique (1) is electrical modulation such asfrequency modulation, amplitude modulation and dither, which isdisclosed in U.S. Pat. No. 5,329,396 December 1994 Fishman et al. Thetechnique (2) relates to control of an optical spectrum using nonlinearphenomenon of an optical fiber, such as Cross-Phase modulation (XPM) andSelf-Phase Modulation (SPM), which is particularly effective to pulselight and disclosed in Y. Horinouchi et al., “Stimulated BrillouinScattering suppression effects induced by cross-phase modulation in highpower WDM repeaterless transmission”, Electron Lettr., Vol. 34, No. 4,pp. 390-391, (1998) and U.S. Pat. No. 6,516,113 B1 April 2003 Glingeneret al.

Besides, in recent years, a demand for large-capacity in the opticalsignal transmission system has been increased. In response to thisdemand for the large capacity, attempts are being made to achieveterabit capacity of the basis transmission system. This terabittransmission system can be achieved mainly by wavelength divisionmultiplexing (WDM) and optical time division multiplexing (OTDM). Theformer is a system such that a plurality of signals of differentwavelengths is multiplexed to be transmitted as a single signal and thelatter is a system of enhancing optical transmission speed (transmissioncapacity per unit time) itself.

In the optical transmission system using these systems, there is amethod of using a repeater for regenerating a signal in an optical areanot electrically in the transmission path (optical regeneration). Insuch optical regeneration, a report has been made as shown in thefollowing Table 1.

TABLE 1 mode-locked fiber laser mode-locked laserdiode (5) required (1)active (2) passive (3) active (4) passive LD + EAM spec. repetition-rate<40 GHz M~GHz <40 GHz >10 GHz <40 GHz >10 GHz temporal >sub-ps<sub-ps >sub-ps >sub-ps >ps <ps duration timing jitter low large lowlarge low low stability low low high high high high synchro. easy noteasy easy easy easy easy

The systems (1), (3) and (5) shown in Table 1 are based on electricalsignal modulation, while the systems (2) and (4) are based on theall-optical technique which needs no electrical circuit. The system (1)is disclosed in M. Nakazawa and E. Yoshida, IEEE Photon. Technol. Lett.,12, 1613 (2000), (2) in K. Tamura et al., Opt. Lett., 18, 1080 (1993),(3) in H. Kuritaetal., IEICE Trans. Electron. E81-C. 129 (1998)., (4) inK. Sato, Electron. Lett., 37, 763 (2001) and (5) in H. Kawatani et al.,OFC2001, MJ3-1 (2001).

Further, as a system other than those shown in Table 1, there isproposed a system of converting dual-frequency light into a solitonpulse train by a soliton compression fiber (E. M. Dianov et al., OL, 141008 (1989)).

Furthermore, in order to realize high-speed signal transmission, thereis a technique of utilizing a light source for generating ahigh-bit-rate optical signal. One example of this technique is anoptical pulse compressor using adiabatic soliton compression, which isdisclosed in “Picosecond soliton pulse-duration-selectable source basedon adiabatic compression in Raman Amplifier” (R. C. Reeves-Hall et al,Electron. Lett., 2000, Vol. 36, pp. 622-624).

From the description made up to this point, the present invention has anobject to provide a light source having an SBS suppressing effect inoptical communication and also a downsized and configurationally simplewaveform shaper, an optical pulse train generator and an opticalregenerating system.

SUMMARY OF THE INVENTION

The present invention relates to a light source in an opticaltransmission system, a waveform shaper, an optical pulse train generatorand an optical regenerating system.

An optical pulse train generator of the present invention includes adual-frequency beat light generating portion, a soliton shaper and asoliton compressor. In the conventional system shown in FIG. 1,dual-frequency beat signal light is lead to the adiabatic solitoncompressor as it is. On the other hand, according to the presentinvention, as shown in FIG. 2, beat light is first subjected toconversion of a sin waveform into a soliton waveform in the solitonshaper before being input to the adiabatic soliton compressor. Thisconsequently allows enhancement of a compression efficiency of theadiabatic soliton compression as well as suppression of SBS (StimulatedBrillouin scattering) in the compression fiber.

Further, the soliton shaper and the soliton compressor on which lightsource performance depends are implemented by a CDPF. The CDPF isconfigured by a combination of a dispersion fiber and a nonlinearityfiber. Average dispersion values of this fiber pair is controlled toallow pulse control. This CDPF technique facilitates optimization ofdispersion profiles of a soliton converter and a soliton compressor. Thelight source of the present invention adopts as one of optimal profilesa profile of increasing average dispersion in the soliton shaper and aprofile of decreasing average dispersion in the soliton compressor.

Furthermore, in order to realize an ideal CDPF, it is important toutilize as a nonlinear fiber a fiber of which the nonlinearitycoefficient is as large as possible. In view of this, it is preferableto use a highly nonlinear fiber (HNLF) having a nonlinearity coefficienthigher than that of a SMF used in transmission. Use of such alow-dispersion HNLF, not only dispersion value but also nonlinearitycoefficient presents comb like profiles, and CDPF which is combinationof a dispersion fiber and a nonlinear fiber can be manufactured ideally.However, it should be noted that interaction of remaining dispersion anddispersion slope and nonlinearity effect results in a noiseamplification phenomenon. In order to suppress this, it is effective touse normal dispersion HNLF. This is because in the normal dispersionarea a parametric gain caused by interaction between the nonlinearityeffect and dispersion is prevented from occurring thereby suppressingnoise amplification.

Furthermore, it is also important to reduce noise at a beat lightgenerating portion. Generally beat light is obtained by multiplexing CWlight outputs from two semiconductor laser diodes (LD). However, LDoutput power is generally the order of tens of milliwatts, which isinsufficient for following soliton shaping and soliton compression.Then, beat light amplification is required, but at this stage noise isalways added. On the other hand, if an LD of high output (≧50 mW) isused, it become possible to realize a beat light generating portion thatdoes not require optical amplification, that is, a beat light generatingportion which has no noise added (FIG. 7). Actually, adistributed-feedback LD of 100 mW output has been developed and by useof this, a low-noise optical beat generating portion can bemanufactured.

In summary, an optical pulse train generator according to the presentinvention is characterized by the following:

(1) Generation of soliton trains from a dual-frequency beat light signalby combination of a CDPF waveform shaper and a CDPF soliton compressor.

(2) Combination of a waveform shaper of dispersion-increasing CDPF witha soliton compressor of dispersion-decreasing CDPF.

(3) Use of a low-dispersion HNLF in a CDPF waveform shaper and a CDPFsoliton compressor.

(4) Noise reduction associated with omission of an EDFA by using two LDsof high outputs.

These result in realization of an adiabatic soliton compressor which hasnot only high quality but also high compression performance, and if sucha compressor is adopted as a light source, it becomes possible togenerate a soliton train of ultra high purity having a repetitionfrequency of more than 100 GHz and time width in subpicosecond range.

On the other hand, this light source can realize synchronization ofsoliton train repetition frequency with an external signal (electric oroptical), which is necessary for application of an optical transmissionsystem. The configuration of this light source which enables externalsynchronization is shown in FIG. 8. The configuration is made bycombination of a light source shown in FIG. 7 and an optical phaselocked loop (OPLL), in which a reference optical signal, which is inthis figure an optical clock pulse train obtained by combination of a LDand a modulator driven by an external electric input signal, and a partof light source output light are compared in repetition frequency in anoptical area. Frequency comparison utilizes four wave mixing (FWM) in anoptical fiber. An LD wavelength of the beat light generating portion isadjusted so that the power of generated FWM light can become maximal.Particularly, as this system utilizes frequency comparison in theoptical area, it should be noted that upper limit thereto is not set.

In addition, the above-described light source is also applicable to anoptical clock pulse light source of an optical regenerating subsystem,which is shown in FIG. 9. After deteriorated transmission signal lightis subjected to waveform shaping, before a clock pulse train generatedbased on a clock frequency extracted from the light signal (clockextraction) is switched by an optical signal. The light source shown inFIG. 8 is applied to this clock extraction and clock pulse traingeneration. The quality of the clock pulse train is extremely importantlight source performance for determining the performance of thetransmission path. In other words, the light source of this inventionenabling generation of a soliton train of extremely high purity issuitable for an optical clock pulse light source in the opticalregenerating system. Further, from the same reason as described above,the light source of this invention is suitable for application to anOTDM-DEMUX portion, which is shown in FIG. 10.

Further, the present invention relates to the technique to suppress SBSin a continuous light source by using a optical fiber nonlinearphenomenon such as XPM, SPM or FWM. Specifically speaking, the presentinvention provides a method and a device for SBS suppression bybroadening the spectrum width of each mode of the multimode continuouslight source by using a nonlinear phenomenon or by reducing power of amode of which the peak power is larger.

The present invention relates to a light source having a further SBSsuppressing effect as compared with the conventional art. Particularly,the present invention allows SBS suppression without increase ofrelative intensity noise (RIN).

In order to cause a nonlinear phenomenon such as XPM or SPM, it isnecessary to change the refractive index corresponding to the opticalintensity caused in a medium having third-order nonlinear sensitivity.As pulse light is relatively apt to generate the effect, pulse light isoften utilized. Therefore, study on continuous light, which is a subjectof the present invention, has not been made.

Further, description of another embodiment of the present invention ismade on a waveform shaper. This waveform shaper can have both functionsof waveform shaper and compression. In other words, if two waveformshapers are connected, waveform shaping and compression can be bothperformed.

One embodiment of the waveform shaper of the present invention is awaveform shaper for converting input light into soliton light, includinga plurality of highly nonlinear optical transmission line segments and aplurality of low nonlinear optical transmission line segments which hasa nonlinearity coefficient lower than that of the plurality of highlynonlinear optical transmission line and a second-order dispersion valueof which an absolute value is different from that of the plurality ofhighly nonlinear optical transmission lines.

According to this embodiment, since the plurality of highly nonlinearoptical transmission line segments and the plurality of low nonlinearoptical transmission line segments are arranged, an equivalentlydispersion-decreasing transmission line can be realized. Since thehighly nonlinear optical transmission line is used, it is possible tomanufacture a waveform shaper having a high dispersion value as a whole.

Another embodiment of the waveform shaper of the present invention isthe waveform shaper of the above-described invention, further includingan optical isolator away from the input end by a distance that is equalto or shorter than a soliton period.

According to this embodiment, since the optical isolator is arrangedaway from the input end by a distance equal to or shorter than thesoliton period, it is possible to prevent SBS in the waveform shaperfrom occurring, thereby outputting soliton light of high intensity.

Another embodiment of the waveform shaper of the present invention isthe waveform shaper of the above-described invention, in which theoptical isolator is arranged at a connecting point of the differentoptical transmission line segments.

According to this embodiment, since the optical isolator is arranged atthe connecting portion of the different transmission path segments, itbecomes possible to decrease the number of connecting portions, therebyreducing an optical loss of the waveform shaper.

Another embodiment of the waveform shaper of the present invention isthe waveform shaper of the above-described invention, in which theoptical isolator is arranged anterior to the highly nonlinear opticaltransmission line segment.

Since the optical isolator is arranged anterior to the highly nonlinearoptical transmission line segment, it is possible to prevent SBS fromoccurring effectively. As the SBS is one nonlinear phenomenon, it oftenoccurs in the highly nonlinear optical transmission line. Accordingly,if the optical isolator is arranged anterior to the highly nonlinearoptical transmission line, SBS occurrence can be suppressed effectively.

One embodiment of an optical pulse generator according to the presentinvention is an optical pulse generator including a highly nonlinearoptical transmission line of 3 km-1.W-1 or more in nonlinearitycoefficient, and comprising pulse width compressing means forcompressing a pulse width by adiabatic soliton compression whileperforming Raman amplification on input light; a pumping light sourcefor providing the pulse width compressing means with pumping light forRaman amplification; and optical coupling means for optically couplingthe pumping light source to the pulse width compressing means.

According to this embodiment, since Raman amplification is performed onlight propagating in the highly nonlinear optical transmission line, thelength of the highly nonlinear optical transmission path can beshortened and the pulse width of output light can be compressed into anarrower width.

Another embodiment of an optical pulse generator according to thepresent invention is an optical pulse generator of the above describedinvention, further including a waveform shaper arranged anterior to thepulse width compressing means and light emitting means arranged anteriorto the waveform shaper.

One embodiment of an optical pulse generator according to the presentinvention is an optical pulse generator of the above describedinvention, further comprising an optical amplifier arranged anterior tothe pulse width compressing means.

One embodiment of an optical pulse generator according to the presentinvention is an optical pulse generator of the above describedinvention, further comprising SBS suppressing means arranged anterior tothe pulse width compressing means.

One embodiment of an optical pulse generator according to the presentinvention is an optical pulse generator including a clock extractingdevice for extracting a repetition frequency of transmitted light; anoptical clock pulse train generating device with a waveform shaper or anoptical pulse generator; and an optical shutter device for modulatinglight output from the optical clock pulse train generating device basedon a frequency extracted by the clock extracting device.

According to this embodiment, as the optical pulse generator is used asan optical clock pulse train generator, it is possible to obtain anoptical clock pulse train with less intensity fluctuation of the opticalpulse and less time fluctuation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a configuration of a conventional systemfor generating high repetition rate soliton trains by solitoncompression of dual-frequency beat light;

FIG. 2 is a view illustrating a configuration of a system for generatinghigh repetition rate soliton trains from dual-frequency beat light,including a combination of soliton shaper and a adiabatic solitoncompressor according to the present invention;

FIG. 3 is a view illustrating a configuration such that in the systemshown in FIG. 2 a dispersion-increasing CDPF is used in a soliton shaperand a dispersion-decreasing CDPF is used in a adiabatic solitoncompressor;

FIG. 4 is a view illustrating a profile of fiber characteristic of asoliton shaping SDPF and a soliton compressing CDPF;

FIG. 5A is a view illustrating autocorrelation traces of input/outputpulses by an optical pulse train generator according to the presentinvention;

FIG. 5B is a view illustrating spectrums of input/output pulses by anoptical pulse train generator according to the present invention;

FIG. 6 is a view illustrating a fiber characteristic profile of asoliton shaping SDPF and a soliton compressing CDPF which shows moremodest dispersion decreasing than that in FIG. 3;

FIG. 7 is a view illustrating a configuration of a beat light generatingportion using high-power DFB-LDs;

FIG. 8 is a view illustrating a light source which is able to be insynchronization with an external signal;

FIG. 9 is a view showing a configuration in which the light source ofthe present invention is applied to an optical clock pulse light sourceof an optical regenerating subsystem;

FIG. 10 is a view illustrating a configuration in which the light sourceof the present invention is applied to an OTDM-DEMUX portion;

FIG. 11 is a view illustrating a transmission path in which a dispersionmedium and a nonlinear medium repeat alternately;

FIG. 12 is a view illustrating an general idea of CPF;

FIG. 13 is a view illustrating a configuration of a waveform shaperusing a Raman amplifier in a HNLF;

FIG. 14 is a graph showing dependence of intensity of input lightrequired for basic soliton pumping on an absolute value of second-orderdispersion value β₂;

FIG. 15 is a graph showing dependence between a limit of output lightpulse width and intensity of input light;

FIG. 16 is a view illustrating a pulse train generator using a waveformshaper which utilizes a Raman amplifier in a HNLF and a CPF;

FIG. 17 is a view illustrating a configuration of an optical pulse traingenerator using a CPF;

FIG. 18 is a view showing a fiber characteristic profile of shaping CDPFand a dispersion-decreasing soliton compressing CDPF;

FIG. 19A is a graph showing autocorrelation traces of input/outputpulses;

FIG. 19B is a graph showing a spectral profile;

FIG. 20 is a view illustrating an optical pulse train generatorincluding a seed pulse generating portion for generating beat light orseed pulses from CW light by using an optical modulator and a pulsewaveform shaper for shortening time width;

FIG. 21 is a graph showing a relationship between a soliton length andan input pulse width;

FIG. 22A is a view illustrating a configuration of a light source forgenerating 40 GHz pulse trains;

FIG. 22B is a graph showing profiles of CDPF dispersion value andnonlinearity coefficient;

FIG. 23A is a graph showing autocorrelation traces of CPF input/outputpulse trains;

FIG. 23B is a graph showing spectral profiles of CPF input/output pulsetrains;

FIG. 24 is a graph showing dependence on input waveform of time width ofCPF output pulse and peal pedestal ratio;

FIG. 25 is a graph showing dependence on CSR of input beat of neighborpulse peak value on autocorrelation traces;

FIG. 26A is a view illustrating a configuration of 40 GHz pulse trainlight source;

FIG. 26B is a graph showing profiles of CDPF dispersion value andnonlinearity coefficient;

FIG. 27A is a graph showing autocorrelation traces of CPF input/outputpulse trains;

FIG. 27B is a graph showing spectral profiles of CPF input/output pulsetrains;

FIG. 28 is a view illustrating an example of an EDFA-free light source;

FIG. 29 is a view illustrating a pulse train generator using a waveformshaper which utilizes a Raman amplifier in a HNLF and a CPF;

FIG. 30 is a graph showing dependence on SCRA input power of SBS opticalpower and SCRA output power with or without CPF;

FIG. 31 is a graph showing dependence on Raman gain and output power ofSCRA output pulse time width;

FIGS. 32A and 32B are views illustrating autocorrelation traces andspectral profiles, respectively, of RA output pulse trains with gains of2.4, 9.4, 11.6 and 12.6 dB;

FIG. 33 is a schematic diagram showing an SBS suppression effect by XPMor SPM;

FIG. 34 is a schematic diagram showing an SBS suppression effect by FWM;

FIG. 35 illustrates an experimental system for examining an SBSsuppression effect at a measured transmission path of a light sourcewhich is subjected to SBS suppression with use of a HNLF;

FIG. 36 illustrates an experimental system for examining an SBSsuppression effect at a measured transmission path of a light sourcewhich is not subjected to SBS suppression with use of a HNLF;

FIG. 37 is a graph showing an SBS measurement result of a 1455 nmsemiconductor laser;

FIG. 38 is a graph showing an SBS measurement result of a 1461 nmsemiconductor laser;

FIG. 39 is a graph showing a spectrum of light from a 1455 nmsemiconductor laser which is made to pass through a HNLF;

FIG. 40 is a graph showing a spectrum of light from a 1455 nmsemiconductor laser which is not made to pass through a HNLF;

FIG. 41 is a graph showing a spectrum of light from a 1461 nmsemiconductor laser which is made to pass through a HNLF;

FIG. 42 is a graph showing a spectrum of light from a 1461 nmsemiconductor laser which is not made to pass through a HNLF;

FIG. 43 is a graph showing a RIN measurement result of the case wherelight from a 1461 nm semiconductor laser is made to pass through an HNLand the case where the light is not made to pass through the HNL;

FIG. 44 is a view illustrating a configuration of a Raman amplifier;

FIG. 45 is a schematic diagram illustrating an SBS suppression lightsource using two continuous light sources;

FIG. 46 shows an experimental system for checking an effect of the SBSsuppression light source using two continuous light sources;

FIG. 47 is a graph showing an SBS measurement result of each lightsource used in the experimental system for checking an effect of the SBSsuppression light source using two continuous light sources;

FIG. 48A is a graph showing a spectral profile of light which was outputfrom an SBS suppression light source using two continuous light sourcesand propagated along a TW-RS fiber;

FIG. 48B is a graph showing a spectral profile of light which was outputfrom an SBS suppression light source using two continuous light sourcesand propagated along a TW-RS fiber;

FIG. 48C is a graph showing a spectral profile of light which was outputfrom an SBS suppression light source using two continuous light sourcesand propagated along a TW-RS fiber;

FIG. 48D is a graph showing a spectral profile of light which was outputfrom an SBS suppression light source using two continuous light sourcesand propagated along a TW-RS fiber;

FIG. 49A is a partially enlarged graph of FIGS. 48A and 48B;

FIG. 49B is a partially enlarged graph of FIGS. 48A and 48C;

FIG. 49C is a partially enlarged graph of FIG. 48D;

FIG. 50 is a schematic diagram illustrating an SBS suppression effect ofan SBS suppression light source using two continuous light sources;

FIG. 51 is a schematic diagram illustrating an SBS suppression lightsource when SBS suppression is performed on plural light sources withwavelengths all together;

FIG. 52 is a schematic diagram illustrating an SBS suppression lightsource when SBS suppression is performed on plural light sources withwavelengths independently;

FIG. 53 is a block diagram illustrating simulation for checking powervariation by FWM;

FIG. 54A shows graphs of simulated relationship of total power and peakpower with fiber length when a fiber of 0/ps/nm/km in dispersion valueat 1460 nm is used;

FIG. 54B shows graphs of simulated relationship of total power and peakpower with fiber length when a fiber of 1/ps/nm/km in dispersion valueat 1460 nm is used;

FIG. 54C shows graphs of simulated relationship of total power and peakpower with fiber length when a fiber of 5/ps/nm/km in dispersion valueat 1460 nm is used;

FIG. 55 shows graphs of relationship between PL_(eff) and peak power;

FIG. 56 is a plot of relationship between PL_(eff) and dispersion whenpeak power falls by 3 dB;

FIG. 57 is a graph showing a relationship between peak power variationand fiber length when wavelength spacing is changed;

FIG. 58A is a schematic illustrating a configuration of a waveformshaper according to the example 11;

FIG. 58B is a graph showing an example of distribution of group velocitydispersion of a waveform shaper according to the example 11;

FIG. 59 is a graph for explaining autocorrelation variation of inputlight and output light when soliton conversion is performed by awaveform shaper according to the example 11;

FIG. 60 is a graph showing an example of distribution of group velocitydispersion of a waveform shaper according to a modification of theexample 11;

FIG. 61 is a schematic diagram illustrating a configuration of awaveform shaper according to the example 12;

FIGS. 62A to 62D are graphs for explaining intensity variation ofreflection light depending on presence of absence of an optical isolatorand on where an optical isolator is arranged;

FIG. 63A is a graph showing autocorrelation of output light;

FIG. 63B is a graph showing a spectral profile of output light;

FIG. 64 is a schematic diagram illustrating a configuration of anoptical pulse generator according to the example 13;

FIG. 65 is a graph showing dependence of intensity of input lightrequired for basic soliton pumping on an absolute value of second-orderdispersion value β₂;

FIG. 66 is a graph showing a relationship between the intensity of inputlight into the pulse compressing transmission path and the limit ofcompression pulse width;

FIG. 67 is a schematic diagram illustrating a configuration of anoptical pulse generator according to a modification of the example 13;

FIG. 68 is a view illustrating a configuration of an opticalregenerating system including a waveform shaper according to the presentinvention; and

FIG. 69 is a schematic view illustrating a configuration of anotherexample.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, preferred embodiments of the presentinvention will be described in detail below.

(Embodiments of an Optical Pulse Train Generator)

First description is made about embodiments of an optical pulse traingenerator according to the present invention. FIGS. 2 and 3 showexamples of an optical pulse train generator according to the presentinvention.

FIG. 2 is a view for illustrating a configuration of the whole system ofthe optical pulse train generator of the present invention. In FIG. 2, awaveform of signal light output from a dual-frequency optical source issin waveform, and a waveform of output light after being shaped by asoliton shaper is sech² waveform that is appropriate for adiabaticsoliton compression. After that, the signal light is made to pass thoughan adiabatic soliton compressor to be a pulse light signal appropriatefor terabit transmission. In the conventional technique, a pulse issoliton-converted in an anterior part of the compressor and compressedin a posterior part thereof. On the other hand, in the presentinvention, a pulse is compressed in an anterior part of the compressorthereby realizing compression ability. Further, the soliton shaper makesit possible to achieve an effective broad spectrum before SBS effectoccurs. As a result of this, the SBS effect of the whole SDPF can bereduced.

FIG. 3 shows an example in which a dispersion-increasing CDPF is appliedto the soliton shaper and a dispersion-decreasing CDPF is adopted to theadiabatic soliton compressor. After light outputs from two semiconductorlasers of different wavelength are combined to be input to anerbium-doped fiber amplifier or semiconductor optical amplifier. Theamplified beat light is input to the two CDPFs. Since these two CDPFsare implemented by combination of the dispersion-decreasing CDPF anddispersion-increasing CDPF, it is possible to realize highcompressibility without reducing pulse quality. In addition, use of CDPFtechnique allows not only easy realization of a dispersion-increasingand dispersion-decreasing profile but also precise and continuouscontrol of a dispersion value by adjusting the fiber length.

Specifically, an optical pulse train generator according to the presentinvention will be implemented by the following:

(1) Dual-frequency light source including two semiconductor lasers foroutputting light signals of different mode and a multiplexer formultiplexing the light signals into dual-frequency signal light.

(2) Soliton shaper for, configured by a dispersion-increasing CDPF,soliton converting the dual-frequency signal light.

(3) Adiabatic soliton compressor, configured by a dispersion-decreasingCDPF, for subjecting the soliton converted signal light to adiabaticsoliton compression.

In addition, in order to generate a soliton train of high quality, it isimportant to use a HNLF (highly nonlinear fiber) of which thenonlinearity coefficient is as large as possible. With a nonlinear fiberimplemented by an ideal CDPF, there occurs only nonlinearity and it isnecessary to cancel dispersion. Conventionally, this nonlinear fiber isoften implemented by a dispersion-shifted fiber of which thesecond-order dispersion value is extremely small. When such a fiber ofextremely small second-order dispersion value is used, higher-orderdispersion effect is enhanced thereby deteriorating the pulse quality.On the other hand, a HNLP of small dispersion value provides not only asmall dispersion effect but also enhanced nonlinearity, and therefore,this fiber can serve as an ideal nonlinear fiber. This consequentlyimproves the freedom of CDPF design and facilitates suppression ofdeterioration of compressed pulse quality.

These dispersion-increasing CDPF and dispersion-decreasing CDPF are eachmanufactured by fusion splicing of a HNKP and a SMF. Accordingly, theseCDPFs can be readily manufactured by adjusting the lengths of these onlytwo kinds of fibers, SMF and HNLP of which a D value is fixed in thelongitudinal direction of the fiber, which results in bringing about agreat merit in actual manufacturing and working. On the other hand, asmentioned above, the conventional dispersion-increasing fiber and theconventional dispersion-decreasing fiber have to be configured by anoptical fiber of which D values are adjusted precisely. Manufacturing ofthese fibers is extremely difficult and manufacturing and constructionactually need much time and costs.

Further, HNLF is a fiber having a minus dispersion value called normaldispersion HNLF. The soliton shaper made of a dispersion-increasing CDPFusing this fiber is able to reduce SBS by a compression fiber therebysuppressing noise. Furthermore, this is compatible with solitoncompressor made of a CDPF using the normal dispersion HNLF, therebyallowing noise-reduced light to be output.

CDPF fiber characteristics are summarized in Table 2.

TABLE 2 D_(ave) HNLF SMF [ps/nm/km] length [m] length [m] 1st pair 3.540 10 2nd pair 5.8 40 20 3rd pair 7.5 40 30 4th pair 3.5 100 25 5th pair1.6 100 10 6th pair 0.8 100 5

Table 2 shows fiber characteristics of a CDPF used in the soliton shaperand adiabatic soliton compressor. D_(ave) indicates an averagedispersion value. In 1^(st) pair through 3^(rd) pair, as the HNLF lengthis fixed at 40 m and as SMF is set longer, the D_(ave) value becomeslarger, which realizes a dispersion-increasing CDPF. This is used in thesoliton shaper.

In 4^(th) pair through 6^(th) pair, as the HNLF length is fixed at 100 mand SMF is set shorter, the D_(ave) value becomes smaller, whichrealizes a dispersion-decreasing SCPF. This is used in the adiabaticsoliton compressor.

Hence, given dispersion-increasing CDPF and dispersion-decreasing CDPFcan be obtained by adjusting lengths of two kinds of fibers, HNLF andSMF.

FIG. 4 shows a fiber profile for waveform shaping and solitoncompression. The upper graph of FIG. 4 shows an ideal dispersionprofile. The vertical axis of the graph indicates dispersion value D inps/nm/km. The horizontal axis indicates a fiber longitudinal distance zin m.

The middle graph of FIG. 4 shows a CDPF dispersion profile of the CDPFof Table 2. The measures of the vertical and horizontal axes are thesame as those in the upper graph of FIG. 4. A broken line shows anaverage dispersion value D_(ave).

The lower graph of FIG. 4 shows a profile of nonlinearity coefficient γof the CDPF of Table 2. The vertical axis of the graph indicatesnonlinearity coefficient γ in 1/w/km and the vertical axis indicates thefiber longitudinal distance z in m.

As is seen from the average dispersion value D_(ave) shown in the middlegraph of FIG. 4, a CDPF for waveform shaping has a predetermineddispersion-increasing profile while a CFPF for adiabatic solitoncompression has a predetermined dispersion-decreasing profile.

It is noted that as is clear from the lower graph of FIG. 4, use of HNLFin CDPF enables realization of a fiber having a comb like profile ofnonlinearity coefficient as well as a comb like profile of fiberdispersion. This makes it possible to obtain an ideal profile in whichthe nonlinearity effect and the dispersion effect are executedalternately.

In other words, if dimensions of a nonlinear medium and a dispersionmedium are adjusted, transmission characteristics can be controlled. Theterm “dimension” used here includes fiber length, medium length,thickness and others that have effects on the nonlinearity anddispersion.

Here, HNLF actual dispersion value and nonlinearity coefficient are 0.8ps/nm/km and 24 l/w/km. Normal dispersion in which the HNLF dispersionvalue presents a minus value makes it possible to suppress parametricgain occurrence during soliton compression, which results in suppressionin noise amplification.

Next, FIGS. 5A and 5B show results of such beat light as described abovewhich is subjected to soliton conversion using a dispersion-increasingCDPF and soliton compression using a dispersion-decreasing CDPF, thatis, input/output pulse waveforms which show performance of the opticalpulse train generator according to the present invention.

In FIG. 5A, autocorrelation traces of input/output pulses are shown byblack dots. The vertical axis of the graph shows optical intensity inarbitrary unit and the horizontal axis of the graph shows time delay inps. An ideal autocorrelation trace of sech² pulses is shown by a brokenline.

FIG. 5B shows optical spectral profiles. The vertical axis of the graphshows a spectrum in 10 dB/div and the horizontal axis of the graph showswavelengths in nm. An ideal sech² waveform is shown by a broken line.

It is noted here that the experimental result and the broken line arematched well not only on the autocorrelation trace but also on theoptical spectrum. Further, as there is no significant change in the modewidth of the CDPF input/output pulse spectrum, noise increase at thesoliton compression stage is proved to be less.

The above described results show that an optical pulse obtained by theoptical pulse train generator according to the present invention is asoliton of ultra high purity. The time width obtained fromautocorrelation fitting is 830 fs.

Accordingly, use of the optical pulse train generator configured bycombining a soliton shaper by dispersion-increasing CDPF using normaldispersion HNLF and an adiabatic soliton compressor bydispersion-decreasing CDPF using normal dispersion HNLF allows to obtainsoliton of ultra high purity compressed into subpicosecond level, whichshows great strides forward practical application of terabittransmission.

In addition, it is proved that if CDPF of modest dispersion decrease ascompared with the CDPF of the example in FIG. 4 is applied, theperformance of soliton compression can be further improved. As anexample, fiber characteristics of a soliton compressor having 6-pairCDPF adiabatic soliton compressing portions are shown in Table 3.

TABLE 3 D_(ave) HNLF SMF [ps/nm/km] length [m] length [m] 1st pair 3.540 10 2nd pair 5.8 40 20 3rd pair 7.5 40 30 4th pair 3.5 100 30 5th pair1.6 100 25 6th pair 0.8 100 20 7th pair 0.8 100 15 8th pair 0.8 100 109th pair 0.8 100 5

D_(ave) indicates an average dispersion value. In 1^(st) pair through3^(rd) pair, as the HNLF length is fixed at 40 m and the SMF is setlonger, the D_(ave) value becomes larger, which realizes adispersion-increasing CDPF. This is used in the soliton shaper.

In 4^(th) pair through 9^(th) pair, as the HNLF length is fixed at 100 mand SMF is set shorter, the D_(ave) value becomes smaller, whichrealizes a dispersion-decreasing CDPF. As compared with those shown intable 2, the length of the dispersion-decreasing CDPF becomes twicelonger. This is used in the adiabatic soliton compressor.

FIG. 6 shows a CDPF fiber characteristic profile. The upper graph ofFIG. 6 shows CDPF dispersion profile shown in Table 3. The vertical axisof the graph indicates a dispersion value D in ps/nm/km and the verticalaxis indicates fiber longitudinal distance z in m.

The lower graph of FIG. 6 shows a profile of the CDPF nonlinearitycoefficient γ of the CDPF shown in table 3. The vertical axis of thegraph shows nonlinearity coefficient γ in l/w/km while the vertical axisindicates the fiber longitudinal distance z in m.

Use of such a CDPF fiber as have the above-described profiles makes itpossible to obtain further compressed soliton of ultra high purity.

It is also important to reduce noise at a beat light generating portion.Generally, beat light is obtained by multiplexing CW light outputs fromtwo semiconductor lasers. However, LD output power is usually in theorder of tens of milliwatts and is insufficient for next-stage solitonconversion and soliton compression. Hence, it is required to amplifybeat light. However, there is a problem of noise of an amplifier.

Among the light source components in FIG. 3, the EDFA causes most noise.In other words, if the EDFA is not used in the system, it is possible togenerate a soliton train of high quality. An example of EDFA-free lightsource based on this idea is shown in FIG. 7. CW light outputs from twohigh-power LDs (≧50 mW) are multiplexed to obtain beat light (≧50 mW).Since this beat light has enough optical power for the next-stagesoliton conversion and compression, no optical amplifier is needed.Accordingly, it is possible to generate a soliton train with less noise.In addition, as an LD driving circuit used in the EDFA can be omitted,downsizing of the device itself is one of the merits.

Next, in consideration of application of a light source according to thepresent invention to an optical transmission system, externalsynchronization of the light source is one indispensable function. Thelight source according to the present invention is characterized byeasily realizing this function.

As an example of this, a configuration of the light source is shown inFIG. 8. This configuration is composed of a combination of a solitontrain generator based on the low noise light source and an OPLL (opticalphase locked loop). The Operation principle of this OPLL is described indetail below.

When external input light is a reference signal, a repetition frequencyof the external input light and a beat frequency of dual-frequency lightare compared in an optical area. This comparison utilizes four wavemixing (FWM) phenomenon which is one of optical fiber nonlinearityeffects. The external light and the beat light are combined and input toa HNLF to generate FWM light. This FWM light is cut off by an opticalfilter to monitor its power. The LD wavelength of the beat lightgenerator is tuned so that this FWM optical power can become maximizedand thereby the frequencies are synchronized. When synchronization ismade with an external electric signal, first, a clock pulse train isgenerated based on this electric signal. This can be suitably carriedout by a simple and high-speed electro-absorption semiconductormodulator (EAM) equipped DFBLD. Further, a HNLF is used to perform pulsecompression if necessary. This optical clock pulse train and therepetition frequency of the beat light are compared in the optical areain the same way as mentioned above to realize synchronization.

Further, it is an important issue in the terabit long-haul transmissionto compensate waveform fluctuations of signal light or a temporaldifference of signal light pulses caused in the long-haul transmissionat a relay point. In this respect, the above-mentioned light sourceaccording to the present invention is applicable to an optical clockpulse light source of the regenerating subsystem, which is shown in FIG.9.

At the first stage of the regeneration, a waveform of deterioratedsignal light can be shaped to some extent by removing noise in thesignal light. This is realized by using a device which has input andoutput profiles with threshold and saturation.

However, this function is not enough to compensate for a temporaldifference of the signal light pulse. Used as the function ofcompensating for a temporal difference of the signal light pulse isretiming, which is described below.

Here, a signal having a timing difference is switched with a clock pulsetrain of reference repetition frequency to realize the retimingfunction. The general method of switching in the optical area is amethod based on the nonlinearity effect of an optical fiber.

It is effective to use an OPLL equipped soliton train generator shown inFIG. 8 in clock pulse generation required for retiming. An output pulseobtained therefrom is a soliton pulse train which is synchronized withthe repetition frequency of input signal light. Since this output pulseis a signal light pulse of high purity, it is extremely appropriate forthe terabit OTDM transmission system for which construction of ahigh-quality transmission path including realization of high-qualitytransmission path and high-quality transmission pulse is an absolutelymust.

Further, a light source according to the present invention is alsosuitable for use in clock pulse train generation at DEMUX portionrequired in an OTDM system receiving portion, which example is shown inFIG. 10.

A system shown in FIG. 10 is such that a transmitted light signal isbranched out and switched with a clock pulse train which has a dividedfrequency of the repetition frequency of the signal. This clock pulsetrain generation and clock extraction utilize the light source shown inFIG. 8. In FIG. 10, the signal is divided by Kerr-shutter, howeverdivision may be performed in another method.

Use of a light source of the present invention makes it possible torealize a DEMUX portion of simple configuration. In addition, in thesame as the regeneration of FIG. 9, noise is removed before switching toshape a waveform, thereby allowing the performance to be furtherenhanced.

As described up to this point, a light source of the present inventionis applicable to every point in the transmission system including areceiving portion, a relay portion and a receiving portion and thepresent invention makes a great contribution to realization of theterabit OTDM transmission system.

Other Embodiments of Optical Pulse Train Generator

Besides, an optical pulse has interesting characteristics in thetemporal, spatial frequency area of which typical examples aretemporally and spatially intensive energy and broadened spectrum. Thereare proposed various applications which utilize these characteristics,and above all, applications to microfabrication, multiphoton absorptionfabrication, sampling measure and optical fiber communication areexpected.

Mainstream of the optical pulse generating method is a large solid laserdevice represented by a mode synchronization titanium sapphire laser.However, since it utilizes the spatial optical system, downsizing isdifficult and stability is poor. These essential problems makeindustrial applicability of this light source thrown into doubt. On theother hand, in terms of downsizing and stability, there is nothing thatexceeds a system using a semiconductor laser diode (LD), or a dielectricand semiconductor modulator. Particularly, this system facilitatessynchronization with an electric signal and essentially hasindispensable characteristics for industrial application. However, atime width of an optical pulse obtained by this system is only the orderof 1 ps, and therefore, an optical pulse compressor is required forfurther time width shortening. Further, output power from one LD is afew hundreds of milliwatts at the maximum and an external opticalamplifier is required for watt or microsiemens amplifier. The presentinvention relates particularly to the former technique of pulsecompression.

As a simple pulse compression system consistent with LD output pulses,there is a system of using optical fiber nonlinearity effect. In thissystem, a method that is particularly excellent in low noise isadiabatic soliton compression. When there exists perturbation inpropagation along an optical fiber, an optical soliton is changed in itswaveform so that a parameter expressed by the following expression,called soliton order, may become constant.

$\begin{matrix}{N = {\sqrt{\frac{\gamma \; P_{0}T_{0}^{2}}{\beta_{2}}} = \sqrt{{1.763\gamma}\frac{ɛ_{p}T_{0}}{\beta_{2}}}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where γ and β₂ are nonlinearity coefficient [l/W/km] and a dispersionvalue [ps²/km], respectively. P₀, T₀ and ∈_(p) are a peak power of theoptical soliton [W], 1/e² pulse width [ps] and pulse energy [pJ]. Whenβ₂ is lowered or ∈_(p) is increased as perturbation, the optical solitondecreases T₀ in order to fix N. This is the principle of adiabaticsoliton compression.

The method for realizing adiabatic soliton compression includes threeways: (1) a dispersion-decreasing fiber (DDF) of which the dispersionvalue is decreased in the fiber longitudinal direction, (2) a fiberimitating a DDF by combining various kinds of fibers and (3) a fiberhaving a modest gain in the longitudinal direction. These ways aresummarized below:

(1) A DPF is an ideal adiabatic soliton compression fiber. However,local dispersion values have to be controlled precisely and itsfabrication is not easy. Dispersion fluctuations cause noiseamplification and lead to deterioration of compression pulse quality.Further, it is necessary to optimize DDF input end dispersion value anddispersion decreasing rate, and therefore, this can not be said to be amethod of high flexibility.

(2) A DDF can be imitated by connecting several kinds of dispersionfixed fibers, which includes a step-like dispersion profiled fiber(SDPF) formed by connecting dispersion-shifter fibers (DSFs) ofdifferent dispersion values and a comb-like dispersion profiled fiberformed by alternately two kinds of DSFs of different dispersion values.These allow precise control of a dispersion value, however, as thereexists fiber splicing loss, these are inferior to other methods in termof compression performance.

(3) Optical fiber amplifiers are broadly divided into an Erbium dopedfiber amplifier (EDFA) and a Raman amplifier. Particularly,amplification of the latter Raman amplifier is such as utilizesstimulated Raman scattering which is nonlinearity optical effect andtherefore, unlike the EDFA, it is possible in the Raman amplifier tocontrol gain amplification by changing intensity and wavelength of pumplight. Accordingly, adiabatic soliton compression is facilitated bycontrolling gain amplification so as to compensate for fluctuations of awave form of input light and DSF dispersion value. However, when anoptical pulse compressor is configured to include a Raman amplifier,there presents a problem of upsizing of the optical pulse compressor.This is because Raman amplification is lower in amplification efficiencythan the EDFA and needs, for enough amplification, a DSF or the likethat has fiber length of several tens of kilometers, which presentsdifficulty in downsizing of the device.

These compression ways have a common problem of Stimulated BrillouinScattering (SBS). SBS is a phenomenon such that there occurs backscattering when an optical wave power reaches some threshold value. Thisphenomenon limits the optical wave power that can be propagated to acompression fiber and makes it difficult to realize effective adiabaticsoliton compression.

Among the above-described three compression fibers, the presentinvention relates to (2) CDPF and (3) Raman amplifier. The presentinvention can be positioned at extension of these ways, however, it isnoted that the present invention presents a completely new concept.Here, it is noted that a waveform shaper used hereinafter means a devicefor shaping a waveform in the temporal and spectral area of opticalwave, including optical pulse compression and spectrum broadeningdevice.

As a typical way of shaping an optical pulse waveform, there is a systemfor controlling an optical wave through a local dispersion effect and alocal nonlinearity effect of a transmission path. This system includesoptical soliton and super continuum light. This system is different fromthe conventional system of tuning a local dispersion effect and a localnonlinearity effect in that the system of the present invention controlsoptical wave by a transmission path in which a dispersion medium and anonlinear medium repeat alternately. A conceptual view of the presentinvention is shown in FIG. 11. Particularly, when phase rotation bydispersion in each dispersion medium and nonlinearity induced phaserotation at each nonlinear medium is at most 1 radian, adjustingdimensions of these media is equal to controlling local dispersion andnonlinearity. The dimensions here include the length of an opticalfiber, the length of a medium, the thickness thereof and every dimensionthat affects nonlinearity and dispersion.

As a method of realizing a transmission path in which a dispersionmedium and a nonlinear medium repeat alternately, there is an opticalfiber according to the present invention. The transmission pathincluding alternately arranged dispersion media and nonlinear media canbe implemented by connecting alternately two kinds of optical fibers ofnot only different dispersion value but also different nonlinearitycoefficient. Such a transmission path is referred to as CPF (Comb-likeprofiled fiber) here. The conceptual view of this CPF is shown in FIG.12. In recent years, a highly-nonlinear fiber (HNLF) is realized thathas nonlinearity coefficient larger by twice or more than that of theconventional fiber. With this fiber, in addition to the nonlinearitycoefficient, dispersion can be controlled like a dispersion-shifterfiber (DSF) so that HNLFs with various dispersion values areimplemented. When such a HNLF is used, it is easy to manufacture a CPF.Needless to say, a fiber other than HNLF, for example a fiber of smallnonlinearity coefficient, can be used to fabricate a CPF.

Conditions of an optical fiber used in this CPF are quantitatively shownby dispersion length LD and nonlinearity length L_(NL). LD and L_(NL)indicate fiber lengths of which phase rotation by dispersion effect andnonlinearity effect becomes 1 radian, which can be expressed by thefollowing equation.

$\begin{matrix}{{L_{D} = \frac{T_{0}^{2}}{\beta_{2}}},{L_{NL} = \frac{1}{\gamma \; P_{0}}}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

where L_(D) and L_(NL) indicate fiber lengths of which phase rotation bydispersion effect and nonlinearity effect, respectively, becomes 1radian. A CPF utilizes a fiber of L_(NL)<L_(D) as a nonlinear medium anda fiber of L_(NL)>L_(D) as a dispersion medium.

Next description is made about an effect of this CPF.

One great advantage of the CPF is to be able to control a localdispersion rate precisely. In general, in order to control a localdispersion value of a fiber, high-level fiber manufacturing technique isessential. On the other hand, a local dispersion rate of the CPF whichhas multiple pairs of a nonlinear medium and a dispersion medium isdetermined by dimension of its dispersion media, in other words, anaverage dispersion value of the pairs. For example, when the CPF iscomposed of a HNLF and an SMF, the local dispersion value of the CPF canbe adjusted precisely by the SMF length, and therefore, control of thelocal dispersion value is extremely easy. This results in allowingaccurate control of optical pulses. For example, a fiber of which adispersion value decreases in the pulse propagating direction and adispersion-increasing fiber can be easily realized by adjusting thelength of the CPS. Further, even a fiber of which a dispersion value inthe longitudinal direction is hard to be stabilized can be manufacturedto be analogue to the fiber with a constant dispersion value by applyingthe CPF to the fiber.

In addition, the CPF has a great advantage of SBS suppression effect.Generally, in the case of fibers which have greatly different dispersionvalues and nonlinearity coefficients, Brillouin gain bands are greatlydifferent. Hence, each fiber used in the CPF has a different Brillouingain band and this decreases the SBS threshold value of the CPF morethan a fiber of the same configuration of the SPF. Since in a CPF with asmall SBS threshold, input power is not limited by the SBS phenomenon,an optical wave can be easily controlled. A method that takes advantageof this characteristic is a method of using a CPF as an SBS suppressor.When the CPF is used for broad spectrum, an operation power conditioncan be set higher.

In addition, an isolator can be inserted to suppress SBS. This isbecause insertion of an isolator contributes to decrease accumulated SBSstimulated backscattering light. Particularly, in a CPF with many fiberconnecting points, it is easy to insert an isolator and the insertionposition can be optimized easily. Generally, when SBS is suppressed inwaveform shaping, an isolator is inserted before the spectrum is widespread. The insertion position of the isolator is preferably set to havea distance from the input end less than the soliton period Z_(soliton).

FIG. 13 shows as another embodiment of the present invention a waveformshaper which utilizes a Raman amplifier using a HNLF. This waveformshaper includes the HNLF, which is a Raman gain medium, a pump LD and aWDM coupler for inputting pump light to the HNLF. This waveforms shaperis characterized by using as a fiber used for Raman gain the HNLF with anonlinearity coefficient of 5 W^(−1 km) ⁻¹ or more. As the HNLF has ahigh nonlinearity coefficient γ, input soliton light can be utilized toperform Raman amplification with a short fiber effectively. Since Ramanamplification is one nonlinearity optical effect, the larger thenonlinearity coefficient T is, the more the amplification efficiency perunit fiber length is enhanced. Accordingly, as the required fiber lengthcan be set shorter, the Raman amplification fiber can be set extremelyshorter, for example, 2 km in length. This is a significant advantage interms of downsizing of the waveform shaper and used of a HNLF canrealize a downsized waveform shaper. Specifically, the Raman gain fibercan be shortened to be one tenth as long as the conventional waveformshaper.

In further, in the waveform shaper according to the present invention,it is possible to, in an absolute value of a predetermined second-orderdispersion value β₂, reduce the intensity of input light more than thatof the conventional waveform shaper. FIG. 14 is a graph showing thedependence of the intensity of input light required for basis solitonpumping with respect to the second-order dispersion value β₂. In FIG. 14the curve line 11 is of a conventional optical pulse compressor whilethe curve line 12 is of the present invention. As is clear from FIG. 14,for example, where |β₂|=1 ps²/km, conventionally, the input light has tobe 150 mW in intensity, however, in the present invention, input lightof optical intensity of around 20 mW is enough for pulse compression tobe performed. When a semiconductor laser device is used as light source,low injection current is sufficient to realize 20 mW output andtherefore, an optical pulse generator according to the present inventioncan do away with an optical amplifier and can be a low-power-consumptionoptical pulse generator.

In addition, when a HNLF is used in a Raman amplification fiber, as theHNLF has a high nonlinearity coefficient γ, the fiber can have a largersecond-order dispersion value β₂ than a fiber of the conventionaloptical pulse compressor. As described above, soliton light to be inputpreferably meets a basic soliton condition, or specifically, N=1 in theequation 1. When the nonlinearity coefficient γ is small, soliton lightto be input of high intensity is required to have a larger second-orderdispersion value β₂. Therefore, it is necessary to reduce second-orderdispersion value β₂. As the nonlinearity coefficient γ of the Ramanamplification fiber of the present invention is high, if thesecond-order dispersion value β₂ is correspondingly set to be high, itis not necessary to increase the intensity of soliton light to be input,thereby allowing highly efficiency pulse compression by input light oflow intensity.

Advantages in increasing a second-order dispersion value β₂ as mentionedabove are described below. Compression of pulse width by adiabaticsoliton compression is fundamentally in accordance with theaforementioned equation (1). However, compression of pulse width isactually limited by a higher-order dispersion value. Specifically, whena third-order dispersion value β₃ and a fourth-order dispersion value β₄are larger than the second-order dispersion value β₂ by respectivepredetermined values, it becomes difficult to perform pulse widthcompression of the soliton light with efficiency. Since in the presentinvention a Raman gain fiber with a higher second-order dispersion valueβ₂ is used and higher-order dispersion values are decreased, it ispossible to eliminate the influence of the higher-order dispersionvalues relatively. Further, it is possible to set the third-orderdispersion value β₃ of the HNLF lower than the conventional one. Forexample, the third-order dispersion value β₃ of the HNLF can be setone-third of the conventional one, that is, only 0.03 ps³/km. Thisallows the pulse width of output light to be compressed into smallervalue.

FIG. 15 is a graph showing a relationship between the intensity of lightinput to the Raman amplifier and the compression pulse width. In FIG.15, the curve line 13 is a curve line of the present invention and thecurve line 14 is of a waveform shaper using a conventional DSF forcomparison. As is clear from the comparison between the curve lines 13and 14, the present invention enables pulse compression into narrowerwidth with the same input light intensity. Specifically, with respect tothe intensity of 10 mW, when the conventional DSF is used, compressionis difficult in only a few ps, while in the waveform shaper of thepresent invention, pulse compression is possible into the order of 200fs.

In addition, when compression is performed into the pulse width of 100fs, the conventional art requires the input light intensity of the orderof 250 mW while the Raman amplification waveform shaper of the presentinvention requires only 46 mW for the nonlinearity coefficient γ=15 W⁻¹km⁻¹ and third-order dispersion value β₃=0.1 ps³/km and 15 mW for thenonlinearity coefficient γ=15 W⁻¹km⁻¹ and third-order dispersion valueβ₃=0.03 ps³/km. Further, in the case of γ=15 W⁻¹km⁻¹ and β₃=0.03 ps³/km,100 fs pulse width can be obtained even with the intensity of 8.3 mW.

Here, if an HNFL is used in the Raman amplification fiber, higher-orderdispersion values can be lower values than those of the conventionalDSF. For example, the third-order dispersion value β₃ is generally theorder of 0.1 ps²/km for the DSF while the third-order dispersion valueβ₃ can be reduced into 0.03 ps²/km for the HNLF. Since the third-orderdispersion value and higher-order dispersion values can be reduced, inthe HNLF, the second-order dispersion value can be relatively higherwith respect to the higher-order dispersion values. For example, whenthe third-order dispersion value of the Raman amplification fiber is0.03 ps²/km, the second-order dispersion value required for pulse widthcompression into 100 fs is 2 ps²/km for the conventional DSF, however,in the HNLF, only 0.6 ps²/km is enough for pulse width compression into100 fs.

Specific examples are described below.

EXAMPLE 1

With use of a waveform shaper using a Raman amplifier in a CPF or a HNLFaccording to the present invention, it becomes possible to realizehigh-performance pulse train generator. The embodiment of this device isshown in FIG. 16. The device is composed of combination including a beatlight generating portion, soliton converter and adiabatic solitoncompressor. The light beat generating portion is a device for generatinga sine wave signal. The beat light is then input to the solitonconverter and subjected to soliton conversion. After soliton conversion,the light is input to the adiabatic soliton compressor and subjectedlow-noise pulse compression. Since the soliton converter and the solitoncompressor are separately provided, optical transmission designs can beapplied to the respective portions. This enables soliton compression ofhigh quality and high efficiency. This soliton converter and theadiabatic soliton compressor may be provided with the aforementioned CPFor Raman amplifier, or with another fiber.

EXAMPLE 2

An optical pulse train generator using a CPF waveform shaper is shown inFIG. 17. This is an embodiment of the light source shown in FIG. 16,including a beat light generating portion composed of two CW-LDs, asoliton converter and a CPF soliton compressor. CW outputs from the twoCW-LDs are combined to obtain beat light with a repetition frequencycorresponding to the wavelength difference. In the first CPF, this beatlight is converted into a soliton which is an optimal waveform for pulsecompression and in the next CPF, the light is compressed. By use of theCPF soliton converter, beat light is not directly compressed but isconverted into appropriate soliton before being compressed therebymaking it possible to realize an ideal adiabatic soliton compressionprocess.

The CPF includes three pairs of HNLF and SMF for each of solitonconversion and soliton compression. The nonlinearity coefficient of theHNLF is 24 l/W/km and its dispersion value is −0.8 ps/nm/km. Here, itshould be note that dispersion of the HNLF is negative normaldispersion. Use of the normal dispersion HNLF allows prevention ofmodulation instability (MI) gain in the HNLF from occurring. Sinceinteraction between noise and optical pulses is enhanced by the MI gain,suppression of MI gain occurrence is important for generation oflow-noise pulse trains.

The CPF dispersion value D profile and CPF nonlinearity coefficient γprofile are shown in FIG. 18( b) and (c). The broken line in FIG. 18( b)is an average dispersion value. First three pairs of the CPF correspondto a soliton converter (shaper) and last three pairs correspond to asoliton compressor. As low-dispersion HNLF and SMF are used in the CPF,both of the dispersion value and the nonlinearity coefficient are likecomb, and a transmission path including alternately arranged nonlinearmedia and dispersion media is realized. It is noted that the dispersionprofile is designed in such a manner that average dispersion values offirst three pairs increase and those of last three pairs decrease. Sincein the CPF the dispersion value of each pair gives an indication of alocal dispersion value, this CPF is equivalent to a combination of adispersion-increasing fiber and a dispersion-decreasing fiber shown inFIG. 18( a). The CPF corresponding to this dispersion-increasing fiberis a soliton converter, where effective conversion from beat light tosoliton can be realized. Adiabatic soliton compression process isrealized in the CPF corresponding to the dispersion-decreasing fiber.The lengths of CPF fibers and pair dispersion values D_(ave) are listedin Table 4 below.

TABLE 4 D_(ave) HNLF SMF [ps/nm/km] length [m] length [m] 1st pair 3.540 10 2nd pair 5.8 40 20 3rd pair 7.5 40 30 4th pair 3.5 100 25 5th pair1.6 100 10 6th pair 0.8 100 5

In addition, an isolator is inserted between each of first and secondpairs and fourth and fifth pairs. This allows SBS suppression in theCPF. Needless to say, the CPF with no isolator inserted also has an SBSsuppression effect, however in order to enhance the SBS suppressioneffect two isolators are inserted. Depending on purposes, the number ofisolators can be increased or decreased.

Results of soliton conversion and compression by CPF, that is, theinputting/outputting pulse waveforms indicating the performance of theoptical pulse train generator according to the present invention areshown in FIGS. 19A and 19B. FIG. 19A shows autocorrelation traces of theinputting/outputting in which the vertical axis indicates the opticalintensity in the unit of a.u. and the horizontal axis indicates delay inthe unit of dB/div. FIG. 19B shows an optical spectrum where thevertical axis indicates a spectrum in the unit of dB/div and thehorizontal axis indicates a wavelength in the unit of mn. Further, aresult of fitting of the pulse waveform into sech² is shown by a brokenline. Here, it is noted that experimental results and the broken lineare matched in the both of the autocorrelation traces and opticalspectrum. Further, as the spectrum width of the CPF inputting/outputtingpulse spectrum is not greatly changed, noise amplification in thesoliton compression step is proved to be less. The time width calculatedfrom the autocorrelation fitting is 830 fs. At this time, a product oftime and bandwidth ΔtΔv is calculated to be 0.34 which shows thatFourier transform limit pulses are obtained. This result shows that anoptical pulse obtained by the optical pulse train generator is solitonof ultra high purity.

EXAMPLE 3

Another embodiment of the present invention is a light source with aconfiguration shown in FIG. 20. This light source is an optical pulsetrain generator including a seed pulse generating portion for generatingbeat light or seed pulses from CW light by using an optical modulator;and a pulse waveform shaper for shortening time width.

A seed pulse is generated by a high-speed optical modulator, forexample, a LiNbO₃ optical modulator (LNM) or an electroabsorptionsemiconductor optical modulator. The high-speed optical modulator, forexample, a LiNbO₃ optical modulator (LNM) or an electroabsorptionsemiconductor optical modulator is driven by an external electric signalto easily generate an optical signal in synchronization with theelectric signal. When an electric pulse signal is used as an electricsignal, an optical pulse like the electric pulse signal is obtained.Needless to say, pulse repetition frequency or pulse time width arelimited by a band of an electronic circuit and presently stay at about40 GHz, >10 ps.

In the present light source, the time width of a seed pulse is shortenedby the pulse waveform shaper. A problem presented when a seed pulsegenerated by an optical modulator is subjected to nonlinearity fiberpulse compression is a fiber length. The pulse time width obtained bythe optical modulator is only 10 ps. Compression of relatively wideoptical pulse needs a compression fiber of tens of kilometers or more.This is because a long fiber is needed in order to have a groupdispersion effect on optical pulses of spectral band of 40 GHz.Generally, in the case of a compressor which utilizes basic solitoncharacteristics, a parameter for determining the fiber length is solitonlength Z_(soliton). Relationship between this value and input pulsewidth Δt is shown in FIG. 21. Here, the fiber dispersion value iscalculated with 10 ps²/km. When Δt increases, Z_(soliton) is increasedin proportion to a square of Δt. For example, Z_(soliton) is 10 km for20 ps pulse compression. In consideration of the adiabatic solitoncompression stage in which the compression rate in a fiber ofZ_(soliton) is 2 or 3, a fiber of at least 20 km is required forcompression of 20 ps pulse into a few ps. Since loss increases in such along fiber, it becomes difficult to realize ideal compression.

On the other hand, as shown in the example 1, in a CPF which hasimproved CDPF performance by optimizing an average dispersion value ofthe pairs and using a HNLF, both of shortening of compression fiber andquality maintaining of compression pulse are realized. This CPF is afiber designed for 160 GHz beat. This CPF is not applicable to 40 GHzbeat or >10 ps optical pulse as it is, however, optimization to inputlight is possible. In general, compression fiber length is in inverseproportion to a square of repetition frequency. In the case of CPF, adispersion fiber, or specifically, a SMF, is only lengthened.Accordingly, for CPF compression, the compression fiber length possiblybecomes the order of several kilometers for >10 ps pulse or 40 GHz beatlight. Actually, the CDPF fiber length for 40 GHz beat light, that is12.5 ps, shown in this example is only 1.8 km.

An embodiment of the present invention is shown in FIG. 22A. Tunablelaser output CW light (wavelength λ_(in)) is subjected to carriersuppression modulation by multielectrode LNM, which is driven by 20 GHzRF signal, to obtain beat light of 40 GHz. When an operating point ofthe LNM is optimized it becomes possible to generate a pulse train witha frequency which is twice larger than that of the driving RF signal.This system is called a carrier suppression modulating system. This 40GHz beat light is amplified to 24 dBm by an EDFA and input to the CPF.Autocorrelation and optical spectrum of the 40 GHz beat light input tothe CPF are shown in FIGS. 23A and 23B. A wavelength resolution of anoptical spectrum analyzer is 0.01 nm. The carrier suppression ratio(CSR) of the beat light is the order of 19 dB. The CPF used here iscomposed of 6 pairs of a HNLF and a SMF. The dispersion value and thenonlinearity coefficient of the HNLF are −0.7 ps/nm/km and 24 l/W/k,respectively. Profiles of CDPF dispersion and nonlinearity coefficientis shown in FIG. 22B. The broken line shows an average dispersion valueof the pairs. As is the case of the example 1, the average dispersionvalues of the first three pairs are increased and average dispersionvalues of the last three pairs are decreased. It should be note thatwith such optimization of the dispersion profile, the compression fiberis allowed to be shortened to 1.8 km even in the system based on theadiabatic soliton compression. The length of each fiber of the CPF islisted in Table 5.

TABLE 5 Fiber length of each pair of CPF length of HNLF length of SMF1st pair  50 m 100 m 2nd pair  50 m 200 m 3rd pair  50 m 300 m 4th pair100 m 500 m 5th pair 100 m 150 m 6th pair 100 m  80 m

FIG. 24 shows λ_(in) dependence of Δt and ΔtΔv of the CPF output pulseand R_(pp). Δt and ΔtΔv are almost constant in the measured range offrom 1530 nm to 1560 nm. R_(pp) is kept >13 dB in the range of from 1530nm to 1560 nm. However, increase in a pedestal amount is shown in theshort wavelength range. This is because SMF dispersion is insufficientin the short wavelength range, in other words, SMF has a positivedispersion slope. From the above-described measurement results, thiscompressor has an operational band of from 1540 to 1560 nm. Here, it isalso noted that there is possibility that the compressor can cover >1560nm band which is out of the measured range.

For tone light of low CSR, a pulse train of 20 GHz can be amplifiedthrough pulse compression. In order to clarify an acceptable CSR of thiscompressor, the CSR dependence of a peak value P_(neigh) of adjacentpulses on the autocorrelation is shown in FIG. 25. For comparison, theexperimental result of dual-frequency CW beat light is plotted by whitedots. As the CSR decreases, the P_(neigh) decrease and the 20 GHz pulsecomponent is grown. For P_(neigh)>0.9 m the CSR is need to be 17 dB ormore. This value is such as easily can be obtained by using themultielectrode LNM.

EXAMPLE 4

Here is described as another embodiment of the present invention anexample of compressing a seed pulse not beat light by a CPF. Aconfiguration view thereof is shown in FIG. 26A. The present inventionincludes a CW laser, an optical modulator and a CPF. CW light is inputto the optical modulator to generate an optical pulse. Here, thehigh-speed LNM is used and driven by 40 GHz sine wave electric signal.However, due to the LNM nonlinear input and output characteristic anoptical output becomes pulses with time width narrower than that of thesine wave. The autocorrelation and optical spectrum are shown in theupper parts of FIGS. 27A and 27B. On the spectrum, there occurs a combcomponent and pulse time width calculated from the autocorrelation is 8ps.

This optical pulse is amplified to 24 dBm by an EDFA and input to theCPF. In this example, last three pairs of CPFs of the example 2 areutilized. First three pairs of CPFs of the example 2 serve to convertbeat light into a soliton train. On the other hand, in thisconfiguration, soliton like optical pulses are already obtained from LNMoutput and therefore, first three pairs of CPFs are not necessary.Profiles of dispersion values D and the nonlinearity coefficients γ areshown in FIG. 26B of the pairs of CFPs. Each of the pairs is summarizedin Table 6.

TABLE 6 CPF Fiber length length of HNLF length of SMF 1st pair 100 m 500m 2nd pair 100 m 150 m 3rd pair 100 m  80 m

Autocorrelation and an optical spectrum waveform of CDPF output pulsesare shown in the lower parts of FIGS. 27A and 27B. They show thatautocorrelation and optical spectrum are matched between experimentalresults and broken lines. The time width calculated from theautocorrelation is 2.3 ps. At the same time, ΔtΔv is 0.32, and Fourierconversion limited pulses.

Among the light source components of FIG. 13, the device that adds mostnoise is an EDFA. In other words, in a system with no EDFA used, asoliton train of higher quality can be generated. Based on this idea anexample of EDFA-free light source is shown in FIG. 28. CW light outputsfrom two LDs of high power output (>50 mW) are combined to be able toobtain beat light (>50 mW). Since this beat light has enough opticalpower for next-stage soliton conversion and compression, no opticalamplifier is necessary. Accordingly, it is possible to generatenoise-less soliton trains. Further, as an LD driving circuit used in tanEDFA can be omitted, the device has the advantage of being downsized.

EXAMPLE 5

Here described is an optical pulse train generator which uses a waveformshaper of soliton compression in Raman amplification (SCRA) in CPF andHNLF. The view of this configuration is FIG. 29. CW light outputs fromtwo DFBLDs are combined by a coupler to obtain dual-frequency 100 GHzbeat light, which is then input to a compression system of the CPF andSCRA. Here, it should be noted that the SPF is arranged before the SCRAin the SCRA fiber in order to suppress SBS. The CPF is composed of sixpairs of HNLF and SMF, of which a dispersion profile is shown in FIG.18. In addition, in order to enhance the SBS suppression effect, twolow-loss optical isolators are inserted.

When this CPF is used to perform soliton-conversion into pulses and thepulses are input to the SCRA, the SBS can be suppressed in the SCRAfiber. In order to confirm this effect, SCRA input power Pin dependenceof SBS optical power P_(SBS), SCRA output power Pout in both cases ofwith use of the CPF and without CPF is shown in FIG. 30. When no CPF isused (upper part of FIG. 30), an SBS threshold value is 15 mW onlywhile, when the SPF is used (lower part of FIG. 30), the SBS thresholdvalue is increased to 40 mW or more.

Then the soliton-converted pulses are compressed by the SCRA. This SCRAis composed of 2.4 km-long low-slope HNLF, counter-pumping LD (outputpower 0.8 W) and two WDM couplers. The nonlinearity coefficient, thedispersion value and the dispersion slope of the HNLF are 25 l/km/W, 1.0ps/nm/km and 0.013 ps/nm²/km, respectively. It should be noted that useof the HNLF shortens the SCRA fiber into several kilo meters order. Ascompared with the conventional art which needs tens of kilo metersorder, the length of the fiber is shortened by one tenth. 7

A pulse input to the SCRA receives Raman gains while it is propagatingalong the HNLF and is subjected to adiabatic soliton compression. Ramangain G and output power Pout dependence of the SCRA output pulse timewidth Δt are plotted by black dots in FIG. 31. The broken line showsPout which is basic soliton pumping power. At the right side of thebroken line, pulse compression is performed and increase in G causesreduction of Δt 2.1 ps to 840 fs. Further, in order to clarify themechanism of this compression, Δt×G is plotted by white dots. As in anare where compression is performed, Δt×G becomes constant, it has beenproved that the present system is based on adiabatic solitoncompression.

FIGS. 32A and 32B show autocorrelation traces of output pulse train andoptical spectrums at gains of 2.4, 9.4, 11.6 and 12.6 dB, respectively.For comparison, sech² waveform is shown by a broken line. Not only theautocorrelation but also the optical spectrum are matched between theexperimental results and broken lines, which shows generation of solitonof high purity. It should be also noted that a ratio of an optical pulsesignal component adjacent to the center wavelength of the opticalspectrum to noise is 40 dB or more. These results have proved that RAoutput pulse is soliton of low in noise and of high purity.

(Embodiments of Continuous Light Source)

As described above, in the optical transmission system, one factor whichlimits input light power into an optical fiber is nonlinear phenomenon.The non linear phenomenon has plural types, one of which is StimulatedBrillouin Scattering (SBS). SBS is apt to occur by relatively smallinput power and a part of the input power is scattered backward toadversely affect the transmission system. The present inventiondiscloses a method for suppressing SBS of continuous light source usingnonlinearity of an optical fiber.

SBS suppression is performed mainly by two methods. One is to broadenthe spectrum width of a light source, and the other is to lessen peakpower of a light source. In the present invention, XPM, SPM andFour-Wave Mixing (FWM) which are nonlinear phenomenon are used tosuppress SBS. Specifically speaking, the present invention provides amethod and a device for suppressing SBS by broadening the spectrum widthof a multimode continuous light source or by reducing peak power withuse of nonlinear phenomenon of an optical fiber.

XPM and SPM have an effect of broadening the spectrum width of a lightsource, which makes it possible to suppress SBS. This is because as thespectrum width is broadened to reduce a Brillouin gain which results inincreasing an SBS threshold value. Its mechanism is illustrated in FIG.33.

In general, in case of continuous light, refractive index is hard tovary by Kerr effect in an optical fiber and these nonlinear phenomenanearly happen. However, if multimode continuous light is used in thelight source, beat light generated between modes can be utilized toeasily cause the nonlinear phenomena. On the other hand, conventionalSBS suppression, pulse light is mainly utilized. However, in the presentinvention, multimode continuous light is applied to a light source, andaccordingly, the nonlinear phenomenon is allowed to occur therebysuppressing SBS.

FIG. 34 is a diagram illustrating SBS suppressing mechanism by use ofFWM. In FIG. 34, the power of multimode power of a light source ischanged to reduce power per mode. As a result of this, a Brillouin gainof each longitudinal mode can be reduced to suppress SBS.

EXAMPLE 6

A continuous light source for suppressing SBS includes a semiconductorlaser for multimode oscillation and a highly nonlinear fiber (HNLF) withzero dispersion wavelength of 1461 nm. Here, the HNLF used here has anonlinearity coefficient γ=24.5/W/km and optical fiber length of 500 m.The longer an optical fiber is and the larger the nonlinearitycoefficient is, the more the nonlinear phenomenon is apt to happenthereby suppressing SBS. However, this is apt to cause SBS in the HNLF.In addition, the longer the HNLF is, the larger the insertion loss is.In consideration of these points, HNLF length and the nonlinearitycoefficient are determined. Further, in order to suppress reflectionlight anterior to or posterior to the HNLF, an isolator may be arrangedaccording to need. The isolator is arranged either of the transmissionpath anterior to the HNLF and the transmission path after the HNLF orboth of them.

FIG. 35 shows a configuration for SBS measurement. At the outputtingside of the HNLF, a 20 dB tap and True Wave RS fiber (nonzero dispersionshifted fiber) care arranged in this order. The TW-RS is 40 km enough tocause SBS. The 20 dB tap is for optically connecting a reflectionmonitor power meter for measuring power of reflection light from theTW-RS and a throughput monitor power for measuring power of input lightwith an SBS measuring transmission path. An SBS value is evaluated by(return loss; SBS value) [dB]=(reflection power)[dBm]−(throughput power)[dBm].

However, in order to clarify an effect for SBS suppression, whenmultimode oscillated continuous light from a semiconductor laser is madeto pass through the HNLF, an SBS value in the measuring transmissionpath (this example) and an SBS value in the measuring transmission pathwithout HNLF shown in the experimental system on FIG. 36 (comparisonexample) are measured. Although an attenuator is provided in theexperimental systems in FIGS. 35 and 36, this is because outputs can bechanged to measure SBS threshold of a semiconductor laser on the samedriving condition.

Description below is made about experimental results of using the SBSmeasuring transmission path on FIGS. 35 and 36.

FIGS. 37 and 38 are graphs showing measuring results of SBS values byusing a semiconductor laser of 1455 nm and a semiconductor laser of 1461nm, respectively. It is confirmed that as the HNLF is used, the SBSthreshold becomes large. Specifically, the SBS suppression effect isremarkable in the 1461 nm semiconductor laser of which the zerodispersion wavelength 1461 nm and the center wavelength of thesemiconductor laser. Generally, the closer to the zero dispersionwavelength of the optical fiber the wavelength of the light source is,FWM is more apt to occur. Accordingly, the wavelength of thesemiconductor laser is preferably tuned to the zero dispersionwavelength.

FIGS. 39 through 42 show spectrums both of when continuous light fromthe 1455 nm and 1461 nm semiconductor lasers is propagated along theHNLF and when the continuous light is not propagated. At this time,light output marking is a value standardized to be the total power. Fromcomparison between the spectrum of a semiconductor laser of 1455 nm withHNLF in FIG. 39 and the spectrum of a semiconductor laser of 1455 nmwithout HNLF in FIG. 40 and from comparison between the spectrum of asemiconductor laser of 1461 nm with HNLF in FIG. 41 and the spectrum ofa semiconductor laser of 1461 nm without HNLF in FIG. 42, the spectrumwith HNLF is broadened by FWM. This is particularly well observed at the1461 nm semiconductor laser for zero-dispersion effect of 1461 nm. Thewavelength range of which this spectrum is broadened is matched with theSBS suppressed wavelength range. In other words, a main factor for SBSsuppression effect is such that the spectrum is broadened by FWM.

FIG. 43 show measuring results of RIN characteristics of both caseswhere semiconductor laser light of waveband which is spectrum broaden byFWM, that is 1461 nm, is propagated along the HNLF and where the lightis not propagated. As is clear from FIG. 43, the measurement results ofthe RIN characteristics confirm that RIN is not affected by FWM. The SBSsuppressing technique by electric control tends to cause deteriorationof RIN in modulated or dithered frequency while the SBS suppressingtechnique by FWM is advantageous in that almost no RIN deteriorationoccurs.

As is seen from the above-described experimental results, when multimodecontinuous light output from a semiconductor laser module is made topass through the HNLF the SBS is suppressed. Besides, when the SBS issuppressed, there occurs almost no RIN deterioration. Chief factor ofSBS suppression is FWM, and therefore the SBS can be suppressedefficiently by controlling the nonlinearity coefficient of the opticalfiber and the optical fiber length in such a manner that a peak value ofeach longitudinal mode is reduced by FWM.

Next description is made about a Raman amplifier using a device and amethod for SBS suppression of a continuous light source as describedabove.

FIG. 44 shows a configuration of the Raman amplifier. The Ramanamplifier is divided into a co-pumping type, a counter-pumping type anda bidirectional type. The co-pumping type Raman amplifier requiresextremely excellent RIN characteristics because in the co-pumping,pumping light and signal light propagate along the optical fibertogether and the RIN characteristic in the pumping light adverselyaffects on the signal light. In other words, the ill RIN characteristicin the pumping light may cause deterioration in the RIN characteristicin the signal light.

Accordingly, there has been developed a semiconductor laser withexcellent RIN characteristic as co-pumping light source of the Ramanamplifier. A pumping light source generally used for Raman amplificationhas Fiber Bragg Grating (FBG) for wavelength fixation. However, FGBtends to deteriorate RIN. Then, in order to improve RIN, a semiconductorlaser with grating inside a semiconductor laser chip is developed so asto fix a wavelength without FBG. However, grating provided inside thechip makes the longitudinal-mode spectrum width narrow, which is apt tocause SBS. Consequently, if the present invention such that RINcharacteristic is not deteriorated even when SBS is suppressed isapplied to a semiconductor laser with grating provided inside, this willbe extremely effective means as forward pumping light source of theRaman amplifier. Further, the Raman amplifier is allowed to solve bothof RIN problem and SBS problem. Although it is more effective forsolving the both problems of RIN and SBS to apply the SBS suppressingmeans of the present invention to the semiconductor laser with gratinginside, the SBS suppressing means of the present invention may beapplied to a semiconductor laser without grating.

EXAMPLE 7

With reference to FIG. 45, description is made about a device and amethod for suppressing SBS of a continuous light source utilizing anonlinear phenomenon of an optical fiber which has a more effective SBSsuppression effect than that of example 6.

The SBS-suppressed continuous light source includes two continuous lightsources, a multiplexer for multiplexing light outputs from the two lightsources, a depolarizer for depolarizing polarization of each of thecontinuous light sources after passing through the multiplexer and anonlinear phenomenon producer for causing nonlinear phenomenon. Here,the depolarizer is not necessarily required. However, when the nonlinearphenomenon producer has polarization dependence, the depolarizer is usedto cause a nonlinear phenomenon more effectively.

Here, at least one of the two continuous light sources is a light sourcefor multimode oscillation. The other light source is either a multimodelight source of which mode frequencies when the two light sources areadded is not equally spaced or a single mode light source. For example,the light sources may be a combination of multimode light sources ofdifferent mode spacing. Or, when two multimode light sources of the samemode spacing are used, the mode frequency of light obtained by additionof the light sources is not set to be equally spaced.

The multiplexer may be a polarization multiplexer. When polarizationmultiplexing is performed, an optical fiber used for outputs from thelight sources and the polarization multiplexer may be preferably apolarization maintaining fiber for effective multiplexing.

The depolarizer may be preferably a crystal type depolarizer or a fibertype depolarizer. For effective multiplexing an optical fiber used forthe polarization multiplexer may be preferably a polarizationmaintaining fiber.

As a parameter indicating the susceptibility to nonlinear phenomenon inan optical fiber, nonlinear phase shift:

Φ=c∫ ₀ ∞P _(s)(z)dz

is generally used. As is expressed by this equation, the larger thenonlinearity coefficient c is or the longer the optical fiber is, thelarger the nonlinear phenomenon becomes. An optical fiber for causingnonlinear phenomenon is such that when the zero dispersion wavelength ofthe optical fiber is near to the wavelengths of the two light sources,FWM, one of nonlinear phenomena, is apt to occur thereby allowingeffective SBS suppression. Further, the longer the fiber is and thelarger the nonlinearity coefficient is, the more the nonlinearphenomenon is apt to occur, thereby allowing effective SBS suppression.

With use of FIG. 46, a specific embodiment of a device and a method forsuppressing SBS of a continuous light source utilizing a nonlinearphenomenon is described. At this time, an optical fiber for transmissionas it is, is used as a nonlinear phenomenon producer. In order tosuppress SBS of a continuous light source in the transmission path, anonlinear phenomenon that occurs in the transmission path is used.TrueWave RS which is a nonzero-dispersion shifted fiber is used as atransmission path. TrueWave RS used here is of 1439.4 nm in zerodispersion wavelength, nonlinear coefficient: γ=2, of 20 km in opticalfiber length. Used as a continuous light source are two semiconductorlasers which oscillate in a multimode in the vicinity of 1442 nm. Theoscillating wavelength of the continuous light source is matched withthe zero dispersion wavelength of the optical fiber so as to easilygenerate FWM which is one nonlinear phenomenon. Used as a polarizationmultiplexer is a polarization maintaining type multiplexer. Used as adepolarizer is a fiber type depolarizer.

FIG. 47 shows SBS measuring result where LD1 and LD2 are drivenseparately. SBS is measured from an input power and a reflected lightpower at an inputting portion of TW-RS. For the LD1, return loss isabout −31 dB between approximately 70 and 170 mW, and the reflectedpower by SBS is less than the Rayleigh scattering level. On the otherhand, for the LD2, the reflected light power by SBS is more thanRayleigh scattering level between about 90 and 190 mW. However, when LD1and LD2 are variously combined to be driven so that the sum of power ofthe LD1 and LD2 becomes 250 mW, the return loss is about −31 dB and thereflection light power by SBS is less than Rayleigh scattering level.That is, both of the LD1 and LD2 are driven to be able to suppress SBS.

FIGS. 48A through 48D show spectrums after TW-RS transmission. FIG. 48Ashows a spectrum when the LD1 is only used to input light into TW-RS at125 mW (condition 1-1). FIG. 48B shows a spectrum when the LD2 is onlyused to input light into TW-RS at 125 mW and the position of alongitudinal mode of the LD2 is adjusted to be little shifted from thatof the LD1 (condition 2-1). FIG. 49A show enlarged views of FIGS. 48Aand 48B. FIG. 48C shows a spectrum when the LD2 is only used to inputlight into TW-RS at 125 mW and the position of a longitudinal mode ofthe LD2 is adjusted to be little shifted from the center of longitudinalmode of the LD1 (condition 2-2). FIG. 49B shows enlarged views of FIGS.48A and 48C. FIG. 48D shows a spectrum when the LD1 and the LD2 are usedat the conditions 1-1 and 2-1, respectively, to input light into TW-RS(condition both-1) and a spectrum when the LD1 and the LD2 are used atthe conditions 1-1 and 2-2, respectively, to input light into TW-RS(condition both-2) FIG. 49C is an enlarged view of FIG. 48D. When FIGS.48A through 48C of the solely driven LD case are compared with FIG. 48Dof the case of two LDs driven simultaneously, the spectrum envelope lineis apparently broadened. When the enlarged view of FIGS. 49A and 49B arecompared with the enlarged view of FIG. 49C, it is seen that the peakpower of the longitudinal mode become smaller by the two LD drivensimultaneously. Particularly, in the condition both-2, the longitudinalmode becomes almost flat. Since the envelop is broadened and peak powerof the longitudinal mode become smaller, there is an effect of SBSsuppression. The envelop is broadened because two LDs are driven toincrease the number of longitudinal modes which tends to cause FWM. Thepeak power of the longitudinal modes becomes smaller because thereoccurs a new wavelength between longitudinal modes of each LD by FWM andfurther FWM is caused between the longitudinal modes of each LD and thenew wavelength thereby distributing the power.

Here, the transmission path itself is used as a nonlinear phenomenonproducer. However, the nonlinear phenomenon producer may be regarded asa part of a continuous light source. If a fiber with a largenonlinearity coefficient is used as a nonlinear phenomenon producer, thesame effect can be achieved even if the fiber length is shortened.

EXAMPLE 8

In order to obtain an SBS suppressed continuous light source of aplurality of wavelengths, SBS suppression can be performed at eachwavelength as shown in FIG. 51, while SBS suppression can be performedafter multiplexing wavelengths as shown in FIG. 52. In multiplexingwavelengths, a WDM may be used or Mach-Zehnder multiplexer may be used.

EXAMPLE 9

In an SBS suppression light source on FIG. 45, in order to obtain fiberrequirements for SBS suppression, FWM is studied by simulations. It isassumed that each continuous light source has eight longitudinal modesand longitudinal mode spacing of about 0.2 nm (35 GHz). Here, sinceadjacent longitudinal modes of each continuous light source areorthogonal to each other by a depolarizer, it is assumed that thereoccurs no FMW in the adjacent longitudinal modes. In other words,simulation is performed on the condition that each continuous lightsource has four longitudinal modes and longitudinal mode spacing is 0.4nm (70 GHz). A configuration of the simulation is shown in FIG. 53.

FIGS. 54A through 54C show simulation results obtained when dispersionat 1460 nm is 0, 1, 5 ps/nm/km. Variation of power at the vertical axisindicates power variation between before and after inputting into HNLF.Total power and peak power are both shown here. As an SBS suppressionlight source, it is effective to reduce peak power while keeping thetotal power. As is seen from figures, the larger the nonlinearitycoefficient is and the longer the fiber is, the more the peak power isapt to be reduced. Besides, as the dispersion is increased, the peakpower is apt to be not reduced much.

In order to estimate an influence of nonlinearity coefficient, power andfiber length on variation of power, graphs with a product PγL_(eff) ofnonlinearity coefficient, power and an effective fiber length (L_(eff))as horizontal axis are shown in FIG. 55. Here, L_(eff)=(1−exp(−α_(p)L))/(α_(p) and α_(p) denotes fiber loss. As shown in FIG. 55,when dispersion is in the vicinity of 0 ps/nm/km, the same behavior canbe obtained in different nonlinearity coefficients, different fiberlengths and different longitudinal power. For example, when thelongitudinal mode power is reduced by 3 dB, an optical fiber of thenonlinearity generator can be designed to be 0.05 in PγL_(eff) or more.When PγL_(eff) is in the range of between 0 and about 0.13, the peakpower is apt to be reduced, and therefore, the optical fiber ispreferably designed to have 0.13 or less PγL_(eff).

In order to estimate an influence of dispersion on variation of power,PγL_(eff) when the peak power is reduced by 3 dB is plotted along thehorizontal axis with dispersion as the vertical axis (see FIG. 56).Reduction of peak power by 3 dB means that power to be input to anoptical fiber in an area where no SBS occurs becomes doubled. As is seenfrom FIG. 56, in an optical fiber of a certain nonlinearity coefficient,when dispersion excesses some value, the peak power is not reduced ifthe PγL_(eff) is increased rapidly. Accordingly, an optical fiber usedas a nonlinearity generator is preferably a fiber with a dispersionvalue less than that corresponding to the rapidly increasing PγL_(eff).Although simulation is not performed for negative dispersion, anabsolute value of dispersion is important and therefore the sametendency as positive dispersion is seen. For example, when thenonlinearity coefficient is 23/W/km, dispersion is set to meet−15<dispersion <15, when the nonlinearity coefficient is 11/W/km,dispersion is set to meet −5< dispersion <5 and when the nonlinearitycoefficient is 5/W/km dispersion is set to meet −2< dispersion <2.

EXAMPLE 10

In order to obtain an interval between two LDs for suppressing SBS in anSBS suppressing light source on FIG. 55, FWM is studied by simulations.The simulation condition is the same as that in example 4 and a fiberused here has at dispersion of 1460 nm a dispersion value of 0 ps/nm/kmand nonlinearity coefficient of 23/W/km. FIG. 57 shows peak powervariation with longitudinal mode 1-m and longitudinal mode 2-m used asparameters. As shown in FIG. 57, when wavelength spacing between two LDsis 0.2 nm (35 GHz) that is one second of longitudinal mode spacing 0.4nm (70 GHz) or 0.10 nm (17.5 GHz) that is one fourth of longitudinalmode spacing 0.4 nm (70 GHz), the peal power does not vary much.However, when the wavelength spacing is 0.05 nm (8.75 GHz) that is oneeighth of longitudinal mode spacing 0.4 nm (70 GHz), the peak power canbe reduced into the same value as obtained when the wavelength spacingbetween LDs 0.4 nm (70 GHz) is not divided by an integer number. (Whenin the case of 0.2 nm, 0.4 nm and 0.8 nm, the wavelength spacing 0.4 nmis divided by an integer number but a frequency can be completelydivided by an integer number) if the wavelength spacing is expressed bydividing the longitudinal mode spacing 0.4 nm (70 GHz) byan integernumber, the longitudinal modes after multiplexing are arranged onequally spaced grids and consequently there occurs no new FWM other thanthe grid. Hence, in order to suppress the peak power, longitudinal modesafter multiplexing are not arranged on equally spaced grids.

(Embodiments of Optical Shaper and Optical Regenerating System)

Next description is made about embodiments of a waveform shaper and anoptical regenerating system. In the figures, the same or like portionsare given the same or like symbols. In addition, the figures areschematic views and it should be noted that the figures are differentfrom actual products. Further, in the figures, there include dimensionsor ratios between the figures.

This waveform shaper is used for both of waveforms shaping andcompression. That is, if two waveform shapers connected, both ofwaveform shaping and compression can be performed.

EXAMPLE 11

First description is made about a waveform shaper according to theexample 11. This waveform shaper is provided for converting input lightinto soliton light. FIG. 58A illustrates a configuration of the waveformshaper and FIG. 58 is a graph showing an example of distribution ofgroup velocity dispersion D of the waveform shaper of the example 11.

The waveform shaper of the example 11 is configured by highly nonlinearoptical transmission lines 1 a, 1 b, 1 c, 1 d and 1 e, for examplecomposed of a highly nonlinear fiber, of which the nonlinearitycoefficient is for example 3 km−1 W−1 or more, or preferably 5.0 km−1W−1, and low nonlinear optical transmission lines 2 a, 2 b, 2 c, 2 d and2 e, for example composed of a single mode fiber, of which thenonlinearity coefficient is for example, 3 km−1 W−1 or less, orpreferably 1 km−1 W−1 or less, the highly nonlinear optical transmissionlines and the low nonlinear optical transmission line being connectedalternately as shown in FIG. 58A. Here, the number of the highlynonlinear optical transmission lines and the number of the low nonlinearoptical transmission paths may be not necessarily five or may be six ormore or four or less.

As shown in FIG. 58B, the highly nonlinear optical transmission lines 1a through 1 e have second-order dispersion values β₂ of which absolutevalues are equivalent values of 4.0 or more. The highly nonlinearoptical transmission lines 1 a through 1 e are configured such thatrespective transmission path lengths 1 a through 1 e satisfyLa>Lb>Lc>Ld. For example, La=100 m, Lb=50 m, Lc=25 m, the transmissionpath lengths are decreased in the longitudinal direction.

The waveform shaper of the example 11 realize a equallydispersion-decreasing fiber by reducing the transmission path lengths ofthe highly nonlinear optical transmission lines 1 a through 1 e in thelongitudinal direction. Specifically, when assuming that the highlynonlinear optical transmission line 1 a and the low nonlinear opticaltransmission line 2 a become a pair and the highly nonlinear opticaltransmission line 1 b and the low nonlinear optical transmission line 2b become a pair, an average of a group velocity dispersion D (−2πc/λ2)β2) is considered, absolute values of the second-order dispersion valueof pairs of the highly nonlinear optical transmission lines and the lownonlinear optical transmission lines are thought to be reduced.Accordingly, when light is input to the waveform shaper of the example1, the light propagates in the waveform shaper and is subjected tosoliton adiabatic compression so that the output light becomes solitonlight.

Since the highly nonlinear optical transmission lines 1 a through 1 eare composed of optical transmission lines of which the nonlinearitycoefficient is for example 3 km−1 W−1 or more, or preferably 5 km−1 W−1,dispersion values can be set higher. Generally, in order to performadiabatic soliton compression, it is necessary to make the nonlinearityproportional to the dispersion in the optical transmission lines.Accordingly, when the waveform shaper is composed of the low nonlinearoptical transmission lines only like in the conventional art, it isnecessary to reduce the second-order dispersion value, however, in thewaveform shaper of the example 11, as the nonlinearity coefficient isincreased, the dispersion, or specifically, the second-order dispersionvalue can be increased correspondingly.

For this reason, with the waveform shaper of the present example 11, itbecomes possible to increase the absolute values of the second-orderdispersion values. As the highly nonlinear optical transmission lines 1a through 1 e with higher absolute values of the second-order dispersionvalues are used, the transmission path lengths can be shortened. Thesoliton period which is a minimum transmission path length that isrequired for theoretically shaping the input light into soliton light isdetermined corresponding to the absolute values of the second-order, andas an absolute vale of the second-order dispersion value becomesincreased, the soliton period becomes smaller. Since the waveform shaperof the example 11 uses highly nonlinear optical transmission lines 1 athrough 1 e, an absolute value of second-order dispersion value can beset larger, which results in that it is possible to realize a downsizedwaveform shaper with the transmission path length shorter all aver thewaveform shaper.

Specifically, an example of a structure of the waveform shaper intowhich pulses of 3 ps with 100 mW peak power is input is described below.Used as the highly nonlinear optical transmission lines 1 a through 1 eis an highly nonlinear fiber of which the nonlinearity coefficient is 15km−1 W−1 and used as the low nonlinear optical transmission lines 2 athrough 2 e is a single mode fiber. These fibers are used to performoptimization and then, the following characteristics of the waveformshaper can be obtained. That is, the nonlinear length is 0.67 km, thedispersion distance is 0.14 km, an average of absolute values ofsecond-order dispersion values is 4.3 ps2/km, and the soliton period is0.25 km. Accordingly, the transmission path length of the whole waveformshaper becomes in the range of between 0.25 km and 0.67 km, which showsthat the waveform shaper of this example can be dramatically shortenedas compared with the conventional waveform shaper of 8 km through 20 km,thereby realizing a downsized waveform shaper.

FIG. 59 is a graph showing correlation change when the waveform shaperwith such a structure. The upper graph of FIG. 59 shows autocorrelationof input light into the waveform shaper and the lower graph of FIG. 59shows autocorrelation of output light from the waveform shaper. As isshown in the lower graph of FIG. 59, although the transmission pathlength is short, the waveform shaper with such a structure as mentionedabove is able to obtain enough soliton light.

Next description is made about advantages of the waveform shaper of thepresent example. The waveform shaper of the example 11 has an advantagesuch that the waveform shaper can be downsized as a whole by shorteningthe entire transmission path length. The waveform shaper has anotheradvantage that the intensity is prevented from decreasing due to anoptical loss by shortening the entire transmission pat length. Althoughan optical fiber usually used as an optical transmission line is anoptical fiber with low loss, when light is propagated along thetransmission path of several kilometers, an optical loss in the opticalfiber becomes not negligible.

However, in the waveform shaper of the example 11, as the propagatingdistance of input light can be reduced into 0.25 km through 0.67 km, theoptical loss can be suppressed into a practically negligible value.

In addition, since the propagating distance of input light is reduced,it becomes possible to suppress another nonlinear effect such as SBSoccurrence more than that in the conventional art. As the SBS occurrenceis suppressed, saturation of the intensity of soliton light to be outputcan be prevented thereby outputting soliton light of high intensity.

Furthermore, since the waveform shaper of the example 11 has aconfiguration of combination of the highly nonlinear opticaltransmission lines and the low nonlinear optical transmission lines, ithas a side advantage. With a highly nonlinear fiber generally used inthe highly nonlinear optical transmission lines, conventionally it isnot easy to perform dispersion control precisely and difficult toconfigure a dispersion-decreasing transmission path by itself. However,in the waveform shaper of this example 11 the transmission path lengthof the entire nonlinear optical transmission lines is controlled torealize equivalently dispersion-decreasing transmission lines, andthereby precise dispersion control can be expected although thenonlinear fiber is used. That is, even if desired dispersioncharacteristic is obtained, the transmission path length can be adjustedthereby to realize equivalent dispersion control.

Modification of Example 11

Next description is made about a modification of a waveform shaperaccording to the example 11. The waveform shaper of this exampleincludes plural highly nonlinear transmission paths having anonlinearity coefficient that is for example 3 km−1W−1 or more, orpreferably 5 km−1 W−1, and different absolute values of the second-orderdispersion values.

FIG. 60 is a graph showing an example of distribution of dispersion D ofthe waveform shaper of this modification. As shown in FIG. 60, thewaveform shaper of this modification is configured to have highlynonlinear optical transmission lines which are sequentially coupled insuch a manner that absolutes of the second-order dispersion values aredecreased along the longitudinal direction. With the waveform shaperwith such a configuration, it is possible to realize an equivalentlydispersion-decreasing transmission path. In addition, since the highlynonlinear optical transmission lines are used, the transmission pathlength can be shortened like in the waveform shaper with a structureshown in FIG. 58A.

EXAMPLE 12

Next description is made about a waveform shaper according to theexample 12. FIG. 61 illustrates a configuration of the waveform shaperof the example 12. In order to suppress SBS, the waveform shaper of theexample 12 is configured to have optical isolators arranged in thewaveform shaper of the example 11. With reference to FIG. 61, thewaveform shaper of this example 12 is described in detail below.

The waveform shaper of the example 12 is configured by highly nonlinearoptical transmission lines 1 a, 1 b, 1 c, 1 d and 1 e, for examplecomposed of a highly nonlinear fiber, of which the nonlinearitycoefficient is 5.0 km−1 W−1, and low nonlinear optical transmissionlines 2 a, 2 b, 2 c, 2 d and 2 e, for example composed of a single modefiber, of which the nonlinearity coefficient is 1 km−1 W−1 or less, thehighly nonlinear optical transmission lines and the low nonlinearoptical transmission lines being connected alternately as shown in FIG.61. In addition, the waveform shaper of this example 12 has opticalisolators 3 a and 3 b arranged at positions away from the input end 4 bydistances less than the soliton period. Specifically, the opticalisolator 3 a is arranged between the low nonlinear optical transmissionline 2 a and the highly nonlinear optical transmission line 1 b and theoptical isolator 3 b is arranged between the low nonlinear opticaltransmission line 2 b and the highly nonlinear optical transmission line1 c. Here, as the highly nonlinear optical transmission lines 1 athrough 1 e and the low nonlinear optical transmission lines 2 a through2 e have the same structures as those in the example 11, descriptionthereof is omitted here.

The optical isolators 3 a and 3 b are provided for suppressing SBSoccurrence inside the waveform shaper. The optical isolators 3 a and 3 bhave function of allowing light propagating in the longitudinaldirection to pass therethrough and cutting off reflected light, and areconfigure by a combination of birefringent crystal, wavelength plate,Faraday rotator and the like.

Next description is made about the reason that the optical isolators 3 aand 3 b are arranged away from the input end 4 by distances that areshorter than the soliton period. The inventors of the presentapplication have measured relationship between the position whereoptical isolator is arranged and the intensity of input light when SBSoccurs (Hereinafter referred to as “SBS threshold intensity”) andoptimized the arranging position of the optical isolator.

First description is made about the waveform shaper of the presentexample 12 which is configured to have only alternately combined highlynonlinear optical transmission lines 1 a through 1 e and low nonlinearoptical transmission lines 2 a through 2 e but no optical isolator. Whenno optical isolator is arranged, the SBS threshold intensity becomes theorder of 50 mW. When the input light intensity becomes 50 mW or more,reflected light by SBS is rapidly increased and the intensity of lightoutput from the waveform shaper becomes saturated. Besides, when anoptical isolator is arranged between the low nonlinear opticaltransmission line 2 d and the highly nonlinear optical transmission line1 e and the SBS threshold intensity is measured, the SBS thresholdintensity is 50 mW like in the case where no optical isolator isarranged. Then, when the waveform shaper is elongated and an opticalisolator is arranged away from the input end 4 by a longer distance, theSBS threshold intensity becomes also 50 mW, which means that the SBSthreshold intensity has not been improved by insertion of the opticalisolator.

On the other hand, when an optical isolator is arranged between the lownonlinear optical transmission line 2 c and the highly nonlinear opticaltransmission line 1 d, SBS threshold intensity is improved to be 75 mW,from which it is clear that the SBS threshold intensity is improved byinsertion of the optical isolator. In further measurement, when anoptical isolator is arranged between the low nonlinear opticaltransmission line 2 b and the highly nonlinear optical transmission line1 c, SBS threshold intensity is 100 mW, and when an optical isolator isarranged between the low nonlinear optical transmission line 2 a and thehighly nonlinear optical transmission line 1 b, SBS threshold intensityis 150 mW. These measurement results show that when an optical isolatoris arranged away from the input end 4 by a distance that is apredetermined value or less, the SBS threshold intensity is improved toa given degree.

From consideration of the insertion position of an optical isolatorwhich brings about improvement of SBS threshold intensity based on thesemeasurement results, it becomes clear that it is effective that theoptical isolator is arranged away from the input end 4 by a distanceshorter than the soliton period. As explained above, the soliton periodis a value corresponding to the transmission path length required forinput light to be converted into soliton light. That is, as SBS alwaysoccurs on the way of the input light being converted into the solitonlight, if an optical isolator is arranged away from the input end 4 by adistance shorter than the soliton period, SBS can be prevented fromoccurring. On the other hand, after input light is converted intosoliton light, there occurs almost no SBS, and if an optical isolator isarranged away from the input end 4 by a distance longer than the solitonperiod, the SBS threshold intensity is hardly improved.

Accordingly, since the optical isolators 3 a and 3 d are arranged awayfrom the input end 4 by distances shorter than the soliton period, theSBS threshold intensity is improved and the waveform shaper of thisexample 12 is allowed to output soliton light of high intensity.

FIGS. 62A to 62D are graphs for explaining the degree of improvement ofthe SBS threshold intensity by arrangement of an optical isolator.Specifically, FIG. 62A shows the intensity of reflected light in awaveform shaper in which no optical isolator is arranged, while FIGS.62B through 62D show the intensity of reflected light when the opticalisolator 3 a is only arranged, the intensity of reflected light when theoptical isolator 3 b is only arranged, and the intensity of reflectedlight when the optical isolators 3 a and 3 b are arranged, respectively.

As is seen from the graph of FIG. 62A, when no optical isolator isarranged, the intensity of reflected light due to SBS is rapidlyincreased in an area where the intensity of input light is 50 mW ormore, that is, the SBS threshold intensity becomes 50 mW. On the otherhand, as shown in FIGS. 62B through 62D, insertion of an opticalisolator contributes to significant improvement of the SBS thresholdintensity. Particularly, when both of the optical isolators 3 a and 3 bare arranged, the SBS threshold intensity is improved to be 200 mW ormore. As is clear from the graph of FIG. 62D, in order to suppress SBSoccurrence effectively, it is preferable that plural optical isolatorsare arranged away from the input end 4 by distances shorter than thesoliton period.

Next, measurement is carried out on optical characteristics of outputlight obtained by the waveform shaper of the present example 12 whenlight of high intensity is input. FIG. 63A is a graph showingautocorrelation traces of the output light, and the upper graph of FIG.63A is of the waveform shaper with no optical isolator while the lowergraph of FIG. 63A is of the waveform shaper of the present example 12.FIG. 63B is a graph showing a spectrum of the output light, and theupper graph of FIG. 63B is of the waveform shaper with no opticalisolator while the lower graph of FIG. 63B is of the waveform shaper ofthe present example 12.

When light of high intensity is input, in the case of a waveform shaperwith no optical isolator, since SBS occurs and the optical intensity inthe optical transmission line is limited. Hence, the optical intensityof propagating light is limited to a given value and enough intensitycan not be obtained. Because of this, soliton conversion can not beperformed sufficiently, which results in as shown in the upper graph ofFIG. 63A and the upper graph of FIG. 63B.

On the other hand, in the waveform shaper of the example 12 sinceoptical isolators are arranged, SBS occurrence during light propagationcan be suppressed. For this reason, there occurs a soliton compressionphenomenon and desired picosecond soliton pulse trains can be obtainedas seen in the lower graph of FIG. 63A and the lower graph of FIG. 63B.

Here, the optical isolator 3 a or the optical isolator 3 b may bearranged at a midway of a highly nonlinear optical transmission line ora low nonlinear optical transmission line. However, they are preferablyarranged at different connecting portions of the optical transmissionline. If they are arranged at the connecting portions, a complicatedprocedure is omitted in the waveform shaper manufacturing method, andfor example, if an optical isolator is formed with both ends ofdifferent optical transmission lines used as both side pigtails, itbecomes possible to reduce the number of fusion splicing points betweenthe different optical transmission lines. There occurs a given opticalloss at a fusion splicing portion between the optical transmissionlines. Since the number of fusion splicing points is reduced, thewaveform shaper of the present example 12 has an advantage in reducingoptical loss in the waveform shaper.

In addition, as shown in FIG. 61, when the waveform shaper is formed bycombination of highly nonlinear optical transmission line segments andlow nonlinear optical transmission line segments, an optical isolator ispreferably arranged anterior to a highly nonlinear optical transmissionline segment. Since SBS is one nonlinear optical phenomenon, SBS oftenoccurs in the highly nonlinear optical transmission line of which thenonlinearity coefficient is high. Therefore, if an optical isolator isarranged anterior to a highly nonlinear optical transmission pathsegment, reflected light occurring in the highly nonlinear opticaltransmission line can be cut off thereby preventing SBS from occurringeffectively.

Here, if an optical isolator is inserted in a waveform shaper accordingto a modification of the example 11, SBS can be suppressed effectively.Also in this case, if an optical isolator is arranged away from theinput end by a distance shorter than the soliton period, it becomespossible to improve SBS threshold intensity and to output soliton lightwith high optical intensity.

EXAMPLE 13

Next description is made about an optical pulse generator according tothe example 13. FIG. 64 us a schematic view illustrating a configurationof the optical pulse generator of the example 13. The optical pulsegenerator can be structurally divided into a soliton light source 6 andan optical pulse compressor 7. The soliton light source 6 is forsupplying soliton light to the optical pulse compressor 7 and theoptical pulse compressor is for performing adiabatic soliton compressionon the input soliton light to compress the pulse width.

The soliton light source includes a light emitting portion 8 foroutputting beat light, for example, and a waveform shaper 9. The opticalpulse compressor 7 includes an input end 10 for inputting light from theoutside, a pulse compressing transmission path 11 connected to the inputend 10 and an output end 12. And, a multiplexer 13 is arranged betweenthe pulse compressing transmission pwth 11 and the output end 12. Apumping light source 14 is connected to the multiplexer 13 so as tosupply pumping light to the pulse compressing transmission path 11.

First description is made about the soliton light source 6. The lightemitting portion 8 of the soliton light source 6 used here in theexample 13 is one that outputs beat light. Specifically, the lightemitting portion 8 has a configuration including a semiconductor laserdevice 15 for outputting laser light of frequency f0, a semiconductorlaser device 16 for outputting laser light of frequency f0+Δf, and amultiplexer 17 for multiplexing laser light outputs from thesemiconductor laser devices 15 and 16. Since the laser light offrequency f0 and the laser light of frequency f0+Δf are combined, thelight emitting portion 8 has a function of outputting beat wave having arepetition period of Δf.

The waveform shaper of the soliton light source 6 may have anystructure, and preferably uses a waveform shaper according to theembodiment 1 or 2. Use of a waveform shaper according to the embodiment1 or 2 allows a downsized device to supply soliton light of high output.

Next description is made about a structure of the optical pulsecompressor 7. First, the pulse compressing transmission path 11 isformed by an optical fiber with abnormal dispersion, that is a positivedispersion value. Further, the pulse compressing transmission path 11has a nonlinearity coefficient γ which is, for example, 3 km−1×W−1 ormore, or preferably, 5.0 km−1 W−1 or more, or more preferably, 15.0km−1W−1. Description below treats the pulse compressing transmissionpath 11 with nonlinearity coefficient γ of 15.0 km−1W−1.

The pumping light source 14 is for supplying pumping light to the pulsecompressing transmission path 11. Specifically, the pumping light source14 is composed of a semiconductor laser device, for example and has afunction of outputting laser light of a predetermined wavelength to thepulse compressing transmission path 11. Raman amplification adopted inthe embodiment 3 is counter-pumping Raman amplification of which thepropagating direction of pumping light is opposed to the propagatingdirection of light amplified. However, Raman amplification adopted heremay be co-pumping Raman amplification of which the propagating directionof pumping light is the same as the propagating direction of lightamplified or bi-directional Raman amplification which is a combinationof co-pumping system and counter-pumping system.

Light output from the pumping light source 14 has a wavelength about 100nm shifted to the short wavelength side with respect to the wavelengthof light to be amplified. This is because in Raman amplification, a peakamplification gain is obtained at a wavelength 100 m shifted to the longwavelength side with respect to the wavelength of the pumping light.

Then, an operation of the optical pulse compressor according to theexample 13 is described. First, soliton light which satisfies apredetermined condition is input to the pulse compressing transmissionpath 11 via the input end from the outside. As an example taken, solitonlight input to the optical pulse compressor of the example 13 satisfiesa basic soliton condition. The input soliton light is subjected toadiabatic soliton compression in the pulse compressing transmission path11 and pulse-compressed light is output to the outside from the outputend 12.

Adiabatic soliton compression in the pulse compressing transmission path11 is described in detail below. Soliton light which satisfies a basicsoliton condition has a property of maintaining the basic solitoncondition, that is, the condition such that the soliton order becomes 1,while propagating in the pulse compressing transmission path 11 evenafter being input the optical pulse compressor of the example 13.Soliton order N is obtained from the following equation (1).

N=(γPT2/|β2|)½  (1)

Here, γ denotes a nonlinearity coefficient of the pulse compressingtransmission path 11, and β2 is a second-order dispersion value of thepulse compressing transmission path 11. P denotes peak intensity ofsoliton light in the pulse compressing transmission path 11 and Tdenotes a pulse width of the soliton light in the pulse compressingtransmission path 11.

Since soliton light propagating along the pulse compressing transmissionpath 11 has a property of maintaining the soliton order 1, the left sidevalue of the equation (1) becomes 1 throughout the pulse compressingtransmission path 11. Besides, in the pulse compressing transmissionpath 11, Raman amplification occurs by the pumping light source 14 whichsupplies pumping light and the peak intensity P of the propagatingsoliton light is increased.

Accordingly, in the right side of the equation (1), the peak intensity Pincreases as the soliton light propagates. On the other hand, in orderto keep the basic soliton condition, N=1 is kept while propagating alongthe pulse compressing transmission path 11, and the nonlinearitycoefficient γ does not vary. In order to keep the equality sign of theequation (1), the pulse width T of the propagating soliton lightdecreases and therefore, compression of the pulse width T is performed.

Since the optical pulse compressor of the present example 13 has a highnonlinearity coefficient γ, Raman amplification subjected to the inputsoliton light can be performed effectively with a short fiber length. AsRaman amplification is one kind of the nonlinear phenomena, the largerthe nonlinearity coefficient γ is, the more amplification efficiency perunit fiber length is enhanced.

This makes it possible to shorten the fiber length required to obtain apredetermined peak intensity P, and the fiber length of pulsecompressing transmission path 11 used for optical pulse compression canbe set to be extremely short, or specifically, the order of 2 km. Thisis an extremely important advantage in term of downsizing of the opticalpulse compressor. Due to this advantage, the pulse compressingtransmission path 11 can be used to realize a downsized optical pulsecompressor. More specifically, it is possible to reduce the length ofthe pulse compressing transmission path 11 by one tenth as compared withthe length of the conventional optical pulse compressor, therebyrealizing a pulse generator with a downsized optical pulse compressor.

In addition, according to the optical pulse compressor 7, it ispossible, with respect to the an absolute value of a given second-orderdispersion value β2, to reduce the intensity of input soliton light ascompared with that of the conventional art. FIG. 65 is a graph showingdependence of the intensity of input light necessary for basic solitonpumping on an absolute value of the second-order dispersion value β2. InFIG. 65, the curve line 11 is of the conventional optical pulsecompressor and the curve line 12 denotes the optical pulse compressor 7of the present example 13.

As is seen from FIG. 65, for example, when |β2|==1 ps2/km is satisfied,the intensity of input light required is about 150 mW for theconventional optical pulse compressor while the intensity of input lightrequired for pulse compression is only about 20 mW for the optical pulsecompressor 7 of the present example 13. As a semiconductor laser deviceused as the light source can easily provide an output of about 20 mWwith low input current, the optical pulse generator of the presentexample 13 does not require an optical amplifier, thereby realizing lowpower consumption in the optical pulse generator.

Further, since the optical pulse compressor 7 of the present example 13has a higher nonlinearity coefficient γ, it is possible to use anoptical fiber with a second-order dispersion value β2 higher than thatof the conventional optical pulse compressor. As described above, inputsoliton light is preferably satisfies the basic soliton condition, ormore specifically, N=1 in the equation (1). Accordingly, when thenonlinearity coefficient γ is small, in order that the second-orderdispersion value β2 may be set larger, the intensity of input solitonlight needs to be high, and therefore, generally it is required toreduce the second-order dispersion value β2. Since the optical pulsecompressor 7 of the present example 13 has a high nonlinearitycoefficient γ, if the second-order dispersion value β2 iscorrespondingly set to be high, the intensity of input soliton lightdoes not need to high, thereby allowing highly-efficient pulsecompression by low-intensity input light.

Next description is made about a merit by a higher second-orderdispersion value β2 in the pulse compressing transmission path 11.Compression of the pulse width by adiabatic soliton compression isbasically performed in accordance with the above-mentioned equation (1),however, it is known pulse width compression is actually limited byhigher-order dispersion values. More specifically, when the third-orderdispersion value β₃ and the fourth-order dispersion value β₄. arepredetermined values or more as compared with the second-orderdispersion value β2, compression of the pulse width of soliton light cannot be performed effectively. When the pulse compressing transmissionpath 11 used has a high second-order dispersion value β₂ as described inthe example 13, higher-order dispersion values are relatively lowered,which makes it possible to eliminate the influence of the higher-orderdispersion values when pulse compression of soliton light is performed.Further, the third-order dispersion value β₃ of the pulse compressingtransmission path 11 can be set lower than that of the conventionaltransmission path and for example, the third-order dispersion value β₃is one third of the conventional transmission path, that is, about 0.03ps3/km. This makes it possible to compress the pulse width of outputlight into a smaller value.

FIG. 66 is a graph showing the relationship between the intensity ofinput light into the pulse compressing transmission path 11 and thelimit of compression pulse width, in which the curve line 13 is of theoptical pulse compressor of the present embodiment 13 and the curve line14 is of the optical pulse compressor using a conventional dispersionshift fiber for comparison. As is clear from the curve lines 13 and 14,the optical pulse compressor of the present embodiment 13 is allowed toperform pulse compression into narrower width range for the same inputlight intensity. Specifically, in the case of the input light intensityof about 10 mW, when the conventional dispersion shift fiber is used,pulse width compression even into a few ps is difficult, while pulsewidth compression into 200 fs is possible for the optical pulsecompressor of the present embodiment 13.

Here, when the pulse width is compressed into 100 fs, the input lightintensity need to be about 250 mW for the conventional fiber, while only46 mW of the intensity is enough for the optical pulse compressor of thepresent embodiment 13 when the nonlinearity coefficient γ satisfies γ=15km−1W−1 and the third-order dispersion value β₃ satisfies β₃=0.1 ps3/km.Further in the optical pulse compressor of the present embodiment 13,when the nonlinearity coefficient γ satisfies γ=15 km−1W−1 and thethird-order dispersion value β₃ satisfies β₃=0.03 ps3/km, only 15 mW ofthe input light intensity is enough, and when the nonlinearitycoefficient γ satisfies γ=25 km−1W−1 and the third-order dispersionvalue β₃ satisfies β₃=0.03 ps3/km, even 8.3 mW of the input lightintensity is enough to achieve 100 fs pulse width.

Here, the pulse compressing transmission path 11 is allowed to have ahigher-order dispersion value lower than that of the conventionaldispersion shift fiber. For example, The third-order dispersion value β₃of the conventional dispersion shift fiber is generally 0.1 ps3/km whilethe third-order dispersion value β₃ of the pulse compressingtransmission path 11 can be reduced to about 0.03 ps3/km. Since thethird-order dispersion value β₂ can be reduced, in the pulse compressingtransmission path 11 it is possible to increase the second-orderdispersion value β₂ relatively as compared with the higher-orderdispersion values. For example, when the third-order dispersion value β₃of the pulse compressing transmission path 11 is 0.03 ps3/km, thesecond-order dispersion value β2 required for pulse width compressinginto 100 fs is about 2 ps2/km for the conventional dispersion shiftfiber, while about 0.6 ps2/km of the second-order dispersion value β2 issufficient for pulse width compressing into 100 fs of the optical pulsecompressor of the present example 13.

Modification of Example 13

Next description is made about a modification of an optical pulsegenerator according to the example 13. FIG. 67 illustrates a structureof an optical pulse generator according to the modification. Inmodification, provided between a soliton light source 6 and an opticalpulse compressor 7 is an SBS Stimulated Brillouin Scattering)suppressing portion 20 for suppressing occurrence of SBS and an opticalamplifier 21. Here, the SBS suppressing portion 20 may be a conventionalone or may be configured by arranging an optical isolator in the opticaltransmission line like in the example 2. Besides, the optical pulsegenerator may be configured such that either the SBS suppressing portion20 or the optical amplifier 21 is arranged.

Up to this point the examples 11 through 13 have been described.However, the present invention is not limited to these examples. Variousmodifications and examples could be conceived by a person skilled in theart. For example, in the examples 11 and 12, another pair of a highlynonlinear optical transmission line and a low nonlinear transmissionpath may be arranged anterior to the input end 4. Here, if the highlynonlinear optical transmission path arranged anterior to the input endis shorter than the highly nonlinear optical transmission path 1 a,soliton conversion can be performed more effectively.

In addition, in the example 13, the waveform shaper 9 and the opticalpulse compressor 7 are arranged independently, however, they may bearranged integrally to be one piece. The pulse compressing transmissionpath 11 making up optical pulse compressor 7 has a high nonlinearitycoefficient thereby to be able to perform adiabatic soliton compressionon beat light. Then, if the transmission path length of the pulsecompressing transmission path 11 is increased, the waveform shaper 9 andthe optical pulse compressor 7 can be integrally formed into one piece.

EXAMPLE 14

Next description is made about an example 14. An optical regeneratingsystem according to the example 14 utilizes a waveform shaper accordingto the example 11 or 12 or an optical pulse generator according to theexample 13. With reference to FIG. 68, the above-mentioned opticalregenerating system using a waveform shaper is described below.

The optical regenerating system 100 shown in FIG. 68 includes anamplifier 102, a multiplexer 103, a clock extracting device 104, anoptical clock pulse train generator 106 composed by an optical pulsegenerator which includes a waveform shaper according to example 1 or 2)(see FIGS. 58 and 62) or an optical pulse generator according to theexample 13 (see FIGS. 64 and 67), and an optical shutter device 108. Theamplifier 102 is for amplifying attenuated signal light and may beconfigured by an Erbium-doped Fiber Amplifier, a Raman amplifier, asemiconductor amplifier, a parametric amplifier or the like.

The clock extracting device 104 is provided for extracting therepetition frequency of a signal light pulse. As is not shown, the clockextracting device 104 is configured by including an electronic circuitbased device composed of, for example, a photo detector, an electricclock extracting circuit and a semiconductor laser. An all-optic clockextracting device may be used which is composed of an amplifier, anonlinear optical medium and optical filter Here, the nonlinear opticalmedium may be for example, highly nonlinear fiber or a semiconductordevice.

The optical clock pulse train generator 106 is provided for generatingan optical clock pulse train with a repetition frequency of a signallight pulse, and utilizes an optical pulse generator which includes awaveform shaper (see FIGS. 58 and 62) or an optical pulse generator (seeFIGS. 64 and 67). Since its configuration, function and the like arealready described above, their description is omitted here.

The optical shutter device 108 is a device for modulating output lightfrom the optical clock pulse train generator 106 by signal light dividedby the multiplexer 103.

In the optical regenerating system 100 of the example 14, transmittedsignal light is amplified by the amplifier 102 and divided by themultiplexer 103. A part of the signal light is propagated as it is to beinput into the optical shutter device 108. The other part of the signallight is input to the clock extracting device 104. An output electricsignal from the clock extracting device 104 is used to control outputlight from the optical clock pulse train generator 106, which is theninput to the optical shutter device 108. In the optical shutter device108, an optical clock pulse train is modulated by the signal lightpropagating from the multiplexer 103 and thereby signal light timing isregenerated.

Thus, when the optical pulse generator of the present invention is usedas the optical clock pulse train generator 106, it becomes possible toobtain optical pulse trains with less fluctuation in optical pulseintensity and time.

(Effects of Waveform Shaper and Optical Regenerating System)

As described above, according to the present invention, as a pluralityof highly nonlinear optical transmission lines and a plurality of lownonlinearity optical transmission lines are arranged and absolute valuesof second-order dispersion values of the highly nonlinear opticaltransmission lines and the low nonlinearity optical transmission pathsare differentiated, it is possible to realize a dispersion-decreasingtransmission path equivalently. Further, as highly nonlinear opticaltransmission lines are use, it becomes possible to realize a waveformshaper with a high dispersion value as a whole.

Further, according to the present invention, as an optical isolator isarranged at a distance that is equal to or less than a soliton period,it is possible to suppress SBS occurrence in the waveform shaper and tooutput soliton light of high intensity.

Furthermore, according to the present invention, as an optical isolatoris arranged at a connecting portion of optical transmission lines, it ispossible to reduce the number of fusion splicing portions and to realizea waveform shaper with reduced optical loss.

Furthermore, according to the present invention, as an optical isolatoris arranged anterior to the highly nonlinear optical transmission line,it is possible to suppress SBS occurrence more effectively. In addition,as SBS is one of nonlinear phenomena, it is apt to occur in a highlynonlinear optical transmission line. Therefore, as an isolator isarranged anterior to the highly nonlinear optical transmission line, SBSoccurrence can be suppressed more effectively.

Furthermore, according to the present invention, as Raman amplificationis performed on light propagating in a highly nonlinear opticaltransmission line, it is possible to shorten the highly nonlinearoptical transmission line and to compress the pulse width of outputlight.

Furthermore, according to the present invention, as a waveform shaper asdescribed above and/or an optical pulse generator as described above isas an optical clock pulse generator making up the optical regeneratingsystem, it is possible to obtain optical clock pulse trains with lessfluctuation in optical pulse intensity and in time.

ANOTHER EMBODIMENT OF THE PRESENT INVENTION

Further, another embodiment of the present invention is described below.

This embodiment relates to a method for forming various optical pulsetrains based on a single frequency light source. Basically, by onesingle frequency light source or after a plurality of single frequencylight sources is combined, signal light with a plurality of frequenciescorrelated with each other is generated. The signal light is dividedinto the correlated frequencies by arrayed waveguide grating, and phasesand intensities of the divided signal light are adjusted before thedivided signal light is combined again by the same type of arrayedwaveguide grating. Since the phase and intensity of each divided signallight, it is possible to obtain a desired pulse train and to make thewaveform of this pulse train vary as appropriate.

The present embodiment is configured as follows.

As shown in FIG. 69, the present invention includes a light source, anonlinear medium for increasing the number of spectral lines, anadjuster for adjusting the phase and the intensity of light per spectralcomponent and a monitoring and controlling portion. Control by theadjuster makes it possible to manipulate the desired waveform flexibly.

In this embodiment, an output of light source having two wavelengths orseveral frequencies with correlation in phase is formed via thenonlinear medium into light with plural frequencies that are spaced by apredetermined distance. At this stage, input light occurs via nonlinearinteraction, and therefore, plural frequencies of generated light arecorrelated in phase with each other. As with this phase correlationmaintained, the phase and the intensity of light of each frequency areadjusted, it becomes possible to multiplex into a flexible pulsewaveform with a repetition frequency period corresponding to the inverseof the frequency period. Time width of the pulse waveform is given bythe inverse of the spectral envelop width broadened by the nonlinearmedium. That is, the present invention is based on free adjustment andcombining of Fourier components of a Fourier-series-developed pulsetrain.

In the conventional art, the waveform of a pulse can no be changedfreely. The waveform is fixed by nonlinearity of a medium, dispersion,loss and its spectrum. Further, when conventional materials are preparedto realize a desired characteristic, there are limits to the spectrum,the length and the like. According to the conventional art, in order toobtain a high-quality pulse waveform, a thermal insulation step isrequired to be used and therefore, there presents a problem that thelength and dimensions are too large for a waveform shaper. In the systemof the present invention, the pulse waveform can be made variable,precise control is possible, less limit is imposed on the spectrum ascompared with the conventional art, and therefore, the system of thepresent invention can be realized advantageously in dimensions, lengthand power consumption. Accordingly, the present invention is superior tothe conventional art in changeability of a pulse waveform, dimensions,power consumption and spectrum.

An example according to the present invention is described below.

EXAMPLE 15

A light source used in this embodiment may be a beat light sourceconfigured by combining two single frequency light sources (variablewavelength may be used) such as DFB lasers having different oscillationwavelengths or may be a pulse light source configured by a singlefrequency light source and an external modulator. The nonlinear mediumused in this example is a PPLN, an optical fiber, a photonic crystal orthe like. The phase and intensity adjuster used in this example includesan arrayed waveguide grating (AWG) using PLC technique, a variableoptical attenuator (VOA) to which the thermo-optic effect is applied,and a phase shifter. The phase shifter may be only able to control aphase delay which is the order of an optical wavelength. As these areprovided on an integrated circuit based on the PLC technique, thestability and the controllability of phase control can be significantlyimproved. If this system is realized in the polarization maintainingsystem, the stability is further improved.

1: A waveform shaper comprising: a Raman gain medium having a nonlinearity coefficient which is 5/W/km or more; a pumping LD; and a coupler for inputting output light from said pumping LD into said Raman gain medium, said waveform shaper being for shaping or compressing an input pulse. 2: An SBS suppressor for suppressing SBS by using at least one of waveform shapers as claimed in claim
 1. 3: A pulse light source comprising: a beat light generating portion; an SBS suppressor for suppressing SBS; and at least one waveform shaper used by the SBS suppressor for suppressing SBS, the waveform shaper comprising: a Raman gain medium having a nonlinearity coefficient which is 5/W/km or more, a pumping LD, and a coupler for inputting output light from said pumping LD into said Raman gain medium, said waveform shaper being for shaping or compressing an input pulse. 